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.     

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.     

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.     

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.     

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.     

Vasopressin antagonizes retrograde amnesia in rats following electroconvulsive shock

Rats given an electroconvulsive shock immediately following training in a passive avoidance task showed amnesia when tested 24 hr after training. This amnesia was prevented if lysine vasopressin was injected either one hr before the training trial or one hr before the first test trial. The results indicate that it is unlikely that either electroconvulsive shock or lysine vasopressin affect memory storage.     

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.     

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.     

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.     

The pituitary-adrenocortical system is not involved in the sex difference in passive avoidance

The hypothesis that sex differences in passive avoidance are related to the sex difference in the pituitary-adrenocortical system was studied. A high dose of dexamethasone (500 microg/kg body weight) was injected in male and female rats in order to suppress the activity of the pituitary-adrenocortical system. Dexamethasone treated animals and controls were tested for retention of passive avoidance at one of 4 different intervals after punishment. The percentage of females re-entering the compartment in which they were previously shocked was significantly higher than the percentage of males, after a retention interval of 60 minutes, but not after an interval of 0 minutes or 15 minutes (Experiment 1). Dexamethasone did not affect this pattern of sex differences. The same sex difference was found after an interval of 24 hours (Experiment 2), and again dexamethasone had no effect on it. However, in males a state-dependent effect of dexamethasone treatment was found in Experiment 2 when animals were given two injections of either dexamethasone or saline, one before the learning trial and one before the retention trial. Within the groups of males given two different injections (Dex-Sal and Sal-Dex) a higher percentage re-entered the shock compartment, when compared with the groups of males given the same injection twice (Sal-Sal and Dex-Dex). Conclusions: (1) A sex difference in passive avoidance apparently occurs after a certain interval during which the animals are not disturbed. (2) This sex difference does not depend on the integrity of the pituitary-adrenocortical system. (3) State-dependency was observed in males only, indicating that changes in the pituitary-adrenocortical system, as a consequence of dexamethasone treatment, may have a more important stimulus value in males.     

Time-related memory effects of vasopressin analogues in rats

The present study was designed to investigate critical time periods for the memory modulating effect of vasopressin and several analogues in rats using a passive avoidance test as the behavioral paradigm. AVP, AVP-(4-8) and AVP-(5-8) were more effective when given immediately after the learning trial (consolidation), while AVP-(1-8) (DGAVP) and AVP-(5-9) were more active when administered one hour prior to the retention test (retrieval). DDAVP and AVP-(4-9) were highly active both when given immediately after the learning trial or 1 hour before the retention test. The period between 12 and 18 hr after the learning trial appeared to be another sensitive period. Administration, in particular of DGAVP, and AVP-(5-9) at 12, 15, and 18 hr after the learning trial induced marked retention of the avoidance response at the 24 hr retention test. Injection at 6 and 21 hr after the learning trial was the least effective in facilitating passive avoidance latencies. The more stable analogue DDAVP facilitated avoidance latencies irrespective of the time of administration. Vasopressin and related peptides exert a long term effect on avoidance behavior. However, DGAVP and AVP-(5-9) facilitated passive avoidance behavior at the 24, 48, and 72 hr retention test if administered immediately after the learning trial. If injection was postponed till 15 hr after the learning trial, passive avoidance behavior was facilitated at the 24 hr retention test only.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of corticosteroids in the hippocampus on passive avoidance behavior in the rat

The site of action of corticosteroids in avoidance learning was investigated in 110 rats. Injection of cycloheximide, 30 min before one-trial training on a passive avoidance task suppressed corticosteroid secretion in response to footshock, and produced an avoidance deficit in a test 6 days later. However, an additional injection of hydrocortisone, either subcutaneously or intra-hippocampally within 5 min of training, restored the avoidance response in the test. Septal and hypothalamic injections of the hormone were ineffective in reversing the cycloheximide effect, whereas the effect of hormone injection into the amygdala was equivocal because of an increased level of activity. Corticosteroids secreted following an aversive experience appear to act upon the steroid-sensitive neurons in the hippocampus to influence the animal's later performance of passive avoidance response.     

Time-dependent retention deficits induced by post-training injections of atropine into the caudate nucleus

Rats were trained on a one-trial passive avoidance task, and retention of the task was measured 24 hr later. Atropine was injected bilaterally into the anterior caudate nuclei (ACN) of rats from independent groups at one of several intervals after training. Application of atropine 2 min after training produced a lack of retention of passive avoidance. An intermediate degree of impairment was seen when the treatment was given 3 min 45 sec after the learning experience, and interference with retention was still noted when an interval of 7 min 30 sec was studied. In contrast, no deficits were observed in groups of animals injected with atropine 15 or 30 min after training. Rats injected with atropine into the parietal cortex 2 min after training showed only a minimal reduction of retention, and a group injected with saline solution into the ACN performed as well as non-treated animals. These results suggest that there is a time-dependent process that mediates the retention of passive avoidance, and that this process requires the activation of cholinergic synapses within the anterior caudate nucleus.     

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.     

ACTH4-9 analog (ORG 2766) facilitates acquisition of an inhibitory avoidance response in rats.

These experiments examined the effects of an ACTH4-9 analog (ORG 2766) on an inhibitory avoidance response in rats. Graded doses of ORG 2766 were administered either 1 hr prior to training, immediately after training, or 1 hr prior to the retention test. The animals were tested 24 hr after training. A 5.0 mg/kg dose was administered prior to training significantly facilitated acquisition of the response. ORG 2766 did not significantly affect retention when administered after training or prior to the retention test. Since ORG 2766 only affected acquisition of the response, it is suggested that the drug acts by influencing sensory, motivationl or attentional variables rather than directly affecting memory consolidation or retrieval processes.     

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.     

Cycloheximide produces adult-like retention deficits of prior learning in infant mice

Utilizing a dosage of cycloheximide which was found to inhibit cerebral protein synthesis by almost 90% after injection, separate groups of 13-day-old mice received either cycloheximide or saline followed by 0 (control), 15, or 25 training trials in a discriminated shock-escape T-maze. Twenty-four hr later, each mouse was treated with cycloheximide or saline and tested for retention by an additional 25 trails in the T-maze. As reflected by correct choice-point turns, the results suggest that whereas salinetreated mice demonstrated reliable retention of prior learning, cycloheximide treated mice exhibited memory impairment; cycloheximide per se had no effect on performance during either original training or retest. A final experiment indicated that this memory impairment was not due to cycloheximide's general debilitating side effects at the time of retention testing. Taken together, these data suggest that protein synthesis inhibition during training impaired consolidation and/or retrieval processes involved in memory. The biochemical and behavioral effects following cycloheximide injection in 13-14-day-old mice in the present study parallel those reported with adult animals and lend indirect support to the hypothesis that the 24-hr memory capacity exhibited by these young mice reflects the early functioning of those processes involved in adult long-term memory.     

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].     

Time variables affecting the permanence of amnesia produced by combined cycloheximide and electroconvulsive shock treatments

Mice pretreated with saline or cycloheximide were trained in a multiple-trial avoidance apparatus. Electroconvulsive shock (ECS) was administered at various intervals following training. The animals were retrained either one day or about one week later. Both transient and permanent memory deficits were observed. Presence and type of memory deficit were dependent upon time of ECS treatment, time of retraining and whether the animals received saline of cycloheximide prior to original training. A possible relationship between the results observed and reported effects of training on macromolecular synthesis in the brain is discussed.     

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.