Protein synthesis and amnesia: studies with emetine and pactamycin

The translational inhibitor and amnestic agent emetine also suppresses ongoing hippocampal neural activity similarly to other blockers of protein synthesis

The consolidation of memory is thought to ultimately depend on the synthesis of new proteins, since translational inhibitors such as anisomycin and cycloheximide adversely affect the permanence of long-term memory. However, when applied directly in brain, these agents also profoundly suppress neural activity to an extent that is directly correlated to the degree of protein synthesis inhibition caused. Given that neural activity itself is likely to help mediate consolidation, this finding is a serious criticism of the strict de novo protein hypothesis of memory. Here, we test the neurophysiological effects of another translational inhibitor, emetine. Unilateral intra-hippocampal infusion of emetine suppressed ongoing local field and multiunit activity at ipsilateral sites as compared to the contralateral hippocampus in a fashion that was positively correlated to the degree of protein synthesis inhibition as confirmed by autoradiography. This suppression of activity was also specific to the circumscribed brain region in which protein synthesis inhibition took place. These experiments provide further evidence that ongoing protein synthesis is necessary and fundamental for neural function and suggest that the disruption of memory observed in behavioral experiments using translational inhibitors may be due, in large part, to neural suppression.

Temporal phases of long-term potentiation (LTP): myth or fact?

Long-term potentiation (LTP) remains the most widely accepted model for learning and memory. In accordance with this belief, the temporal differentiation of LTP into early and late phases is accepted as reflecting the differentiation of short-term and long-term memory. Moreover, during the past 30 years, protein synthesis inhibitors have been used to separate the early, protein synthesis-independent (E-LTP) phase and the late, protein synthesis-dependent (L-LTP) phase. However, the role of these proteins has not been formally identified. Additionally, several reports failed to show an effect of protein synthesis inhibitors on LTP. In this review, a detailed analysis of extensive behavioral and electrophysiological data reveals that the presumed correspondence of LTP temporal phases to memory phases is neither experimentally nor theoretically consistent. Moreover, an overview of the time courses of E-LTP in hippocampal slices reveals a wide variability ranging from <1 h to more than 5 h. The existence of all these conflictual findings should lead to a new vision of LTP. We believe that the E-LTP vs. L-LTP distinction, established with protein synthesis inhibitor studies, reflects a false dichotomy. We suggest that the duration of LTP and its dependency on protein synthesis are related to the availability of a set of proteins at synapses and not to the de novo synthesis of plasticity-related proteins. This availability is determined by protein turnover kinetics, which is regulated by previous and ongoing electrical activities and by energy store availability.

The role of protein synthesis in memory consolidation: progress amid decades of debate

A major component of consolidation theory holds that protein synthesis is required to produce the synaptic modification needed for long-term memory storage. Protein synthesis inhibitors have played a pivotal role in the development of this theory. However, these commonly used drugs have unintended effects that have prompted some to reevaluate the role of protein synthesis in memory consolidation. Here we review the role of protein synthesis in memory formation as proposed by consolidation theory calling special attention to the controversy involving the non-specific effects of a group of protein synthesis inhibitors commonly used to study memory formation in vivo. We argue that molecular and genetic approaches that were subsequently applied to the problem of memory formation confirm the results of less selective pharmacological studies. Thus, to a certain extent, the debate over the role of protein synthesis in memory based on interpretational difficulties inherent to the use of protein synthesis inhibitors may be somewhat moot. We conclude by presenting avenues of research we believe will best provide answers to both long-standing and more recent questions facing field of learning and memory.

Post-translational protein modification as the substrate for long-lasting memory

Prevailing models of memory identify mRNA translation as necessary for long-lasting information storage. However, there are enough instances of memory storage in the virtual absence of protein synthesis to prompt consideration of alternative models. A comprehensive review of the protein synthesis literature leads us to conclude that the translational mechanism is exclusively a permissive, replenishment step. Therefore, we propose that post-translational modification (PTM) of proteins already at the synapse is the crucial instructive mechanism underlying long-lasting memory. A novel feature of this model is that non-random spontaneous (or endogenous) brain activity operates as a regulated positive-feedback rehearsal mechanism, updating network configurations by fine-tuning the PTM state of previously modified proteins. Synapses participating in memory storage are therefore supple, a feature required for networks to alter complexity and update continuously. In analogy with codons for amino acids, a long-lasting memory is represented by a 'degenerate code' - a set of pseudo-redundant networks that can ensure its longevity.

Protein synthesis inhibitors delay transneuronal death in the piriform cortex of young adult rats

It has been demonstrated that apoptotic cell death is an active process that is dependent on RNA and protein synthesis. The question remains as to whether neuronal death in adult, mammalian brains can also be demonstrated in vivo to be dependent on protein synthesis. To address this question we have analysed transneuronal death in the piriform (olfactory) cortex. Following unilateral olfactory bulb ablation in young adult rats, layer IIa of the piriform cortex undergoes rapid degeneration, that commences 12 h after ablation and that is almost complete at 48 h. In order to block protein synthesis, three to six subcutaneous injections of the short acting protein synthesis inhibitor anisomycin, were given at 2 h intervals beginning just before the ablation of the olfactory bulb. In other cases a single injection of the long acting protein synthesis inhibitor emetine were made intracerebrally just before or after olfactory bulb ablation. The number of dying cells was then counted in sections through the rostrocaudal extent of the piriform cortex. Both anisomycin and emetine injections markedly reduced the number of pyknotic cells in layer IIa of the piriform cortex after olfactory bulb ablation. The effect of anisomycin was dose-dependent, near lethal doses leading to an almost complete absence of cell death (six injections of 100 mg/kg). As the doses of anisomycin were reduced, more dying cells were observed. Emetine was only effective at near lethal doses (10 mg/kg) and showed a greater capacity to reduce the levels of cell death when injected into structures near the piriform cortex (e.g., accumbens nucleus) than when injected into more distant structures. To further confirm that the cell death observed was due to apoptosis, we analysed sections by tunel staining to demonstrate DNA fragmentation. We found that tunel-positive cells were also always pyknotic, one of the landmarks of apoptosis. The appearance of pyknotic cells labelled by the tunel method demonstrated that the dying cells in the piriform cortex did indeed undergo apoptosis.

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 rr 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.

Biochemical effects of cycloheximide in developing chick brain

The timecourses of inhibition of protein synthesis in forebrain roof 15 min to 24 hr after either intracerebral or peripheral injection of 40 microgram of cycloheximide (CXM), show maximum inhibition (68--85%) 1 hr after injection in 12 hr, 2-day- and 16-day-old chicks. In 2-day-old chicks, the level of free lysine was elevated by around 250% 1 hr after intracerebral injection of CXM. The total radioactivity in the forebrain roof/mg protein following peripheral injection of labelled lysine increased by around 80% compared to saline controls. Following unilateral injection of 20 microgram in 25 microliter CXM into a forebrain hemisphere, the inhibition of protein synthesis spread to the opposite forebrain hemisphere and both optic lobes within 5 min. Smaller volumes did not seem to give a clear unilateral inhibition capable of explaining the unilateral behavioural differences which have been reported. Central injection of CXM in 2- and 16-day-old chicks, resulted in an inhibition of liver protein synthesis of 55% and 40%, respectively. There were also gross signs of pathological effects in the liver and gall bladder. The implications of these effects of CXM are discussed in terms of the use of the drug in behavioural experiments.