Attenuation of experimentally-induced amnesia

The molecular memory code and synaptic plasticity: A synthesis

The most widely accepted view of memory in the brain holds that synapses are the storage sites of memory, and that memories are formed through associative modification of synapses. This view has been challenged on conceptual and empirical grounds. As an alternative, it has been proposed that molecules within the cell body are the storage sites of memory, and that memories are formed through biochemical operations on these molecules. This paper proposes a synthesis of these two views, grounded in a computational model of memory. Synapses are conceived as storage sites for the parameters of an approximate posterior probability distribution over latent causes. Intracellular molecules are conceived as storage sites for the parameters of a generative model. The model stipulates how these two components work together as part of an integrated algorithm for learning and inference.

Memory Integration as a Challenge to the Consolidation/Reconsolidation Hypothesis: Similarities, Differences and Perspectives

We recently proposed that retrograde amnesia does not result from a disruption of the consolidation/reconsolidation processes but rather to the integration of the internal state induced by the amnesic treatment within the initial memory. Accordingly, the performance disruption induced by an amnesic agent does not result from a disruption of the memory fixation process, but from a difference in the internal state present during the learning phase (or reactivation) and at the later retention test: a case of state-dependency. In the present article, we will review similarities and differences these two competing views may have on memory processing. We will also consider the consequences the integration concept may have on the way memory is built, maintained and retrieved, as well as future research perspectives that such a new view may generate.

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.

Cognitive neuroepigenetics: the next evolution in our understanding of the molecular mechanisms underlying learning and memory?

A complete understanding of the fundamental mechanisms of learning and memory continues to elude neuroscientists. Although many important discoveries have been made, the question of how memories are encoded and maintained at the molecular level remains. To date, this issue has been framed within the context of one of the most dominant concepts in molecular biology, the central dogma, and the result has been a protein-centric view of memory. Here we discuss the evidence supporting a role for neuroepigenetic mechanisms, which constitute dynamic and reversible, state-dependent modifications at all levels of control over cellular function, and their role in learning and memory. This neuroepigenetic view suggests that DNA, RNA and protein each influence one another to produce a holistic cellular state that contributes to the formation and maintenance of memory, and predicts a parallel and distributed system for the consolidation, storage and retrieval of the engram.

Protein synthesis and consolidation of memory-related synaptic changes

Although sometimes disputed, it has been assumed for several decades that new proteins synthesized following a learning event are required for consolidation of subsequent memory. Published findings and new results described here challenge this idea. Protein synthesis inhibitors did not prevent Theta Bust Stimulation (TBS) from producing extremely stable long-term potentiation (LTP) in experiments using standard hippocampal slice protocols. However, the inhibitors were effective under conditions that likely depleted protein levels prior to attempts to induce the potentiation effect. Experiments showed that induction of LTP at one input, and thus a prior episode of protein synthesis, eliminated the effects of inhibitors on potentiation of a second input even in depleted slices. These observations suggest that a primary role of translation and transcription processes initiated by learning events is to prepare neurons to support future learning. Other work has provided support for an alternative theory of consolidation. Specifically, if the synaptic changes that support memory are to endure, learning events/TBS must engage a complex set of signaling processes that reorganize and re-stabilize the spine actin cytoskeleton. This is accomplished in fast (10 min) and slow (50 min) stages with the first requiring integrin activation and the second a recovery of integrin functioning. These results align with, and provide mechanisms for, the long-held view that memories are established and consolidated over a set of temporally distinct phases. This article is part of a Special Issue entitled SI: Brain and Memory.

Cycloheximide impairs and enhances memory depending on dose and footshock intensity

This experiment examined the effects on memory of interactions of cycloheximide dose and training foot shock intensity. Mice received injections of cycloheximide (120 mg/kg, s.c.) or saline 30 min prior to inhibitory avoidance training with shock intensities of 100, 150, 250 or 300 μA (1 s duration). Memory was tested 48 h later. The saline control mice showed increasing memory latencies as a function of shock intensity. The ability of cycloheximide to impair memory increased as the training shock intensity increased. In a second experiment, mice were trained with a 200 μA (1 s duration) shock and received injections of saline or cycloheximide at one of several doses (30, 60 or 120 mg/kg). Under these training conditions, cycloheximide enhanced memory in an inverted-U dose-response manner. These findings are consistent with prior findings suggesting that protein synthesis inhibitors act on memory by altering modulators of memory formation as a secondary consequence of the inhibition of protein synthesis rather than by interfering with training-initiated synthesis of proteins required for memory formation.

Lidocaine attenuates anisomycin-induced amnesia and release of norepinephrine in the amygdala

When administered near the time of training, protein synthesis inhibitors such as anisomycin impair later memory. A common interpretation of these findings is that memory consolidation requires new protein synthesis initiated by training. However, recent findings support an alternative interpretation that abnormally large increases in neurotransmitter release after injections of anisomycin may be responsible for producing amnesia. In the present study, a local anesthetic was administered prior to anisomycin injections in an attempt to mitigate neurotransmitter actions and thereby attenuate the resulting amnesia. Rats received lidocaine and anisomycin injections into the amygdala 130 and 120 min, respectively, prior to inhibitory avoidance training. Memory tests 48 h later revealed that lidocaine attenuated anisomycin-induced amnesia. In other rats, in vivo microdialysis was performed at the site of amygdala infusion of lidocaine and anisomycin. As seen previously, anisomycin injections produced large increases in release of norepinephrine in the amygdala. Lidocaine attenuated the anisomycin-induced increase in release of norepinephrine but did not reverse anisomycin inhibition of protein synthesis, as assessed by c-Fos immunohistochemistry. These findings are consistent with past evidence suggesting that anisomycin causes amnesia by initiating abnormal release of neurotransmitters in response to the inhibition of protein synthesis.

Intrahippocampal infusions of anisomycin produce amnesia: contribution of increased release of norepinephrine, dopamine, and acetylcholine

Intra-amygdala injections of anisomycin produce large increases in the release of norepinephrine (NE), dopamine (DA), and serotonin in the amygdala. Pretreatment with intra-amygdala injections of the beta-adrenergic receptor antagonist propranolol attenuates anisomycin-induced amnesia without reversing the inhibition of protein synthesis, and injections of NE alone produce amnesia. These findings suggest that abnormal neurotransmitter responses may be the basis for amnesia produced by inhibition of protein synthesis. The present experiment extends these findings to the hippocampus and adds acetylcholine (ACh) to the list of neurotransmitters affected by anisomycin. Using in vivo microdialysis at the site of injection, release of NE, DA, and ACh was measured before and after injections of anisomycin into the hippocampus. Anisomycin impaired inhibitory avoidance memory when rats were tested 48 h after training and also produced substantial increases in local release of NE, DA, and ACh. In an additional experiment, pretreatment with intrahippocampal injections of propranolol prior to anisomycin and training significantly attenuated anisomycin-induced amnesia. The disruption of neurotransmitter release patterns at the site of injection appears to contribute significantly to the mechanisms underlying amnesia produced by protein synthesis inhibitors, calling into question the dominant interpretation that the amnesia reflects loss of training-initiated protein synthesis necessary for memory formation. Instead, the findings suggest that proteins needed for memory formation are available prior to an experience, and that post-translational modifications of these proteins may be sufficient to enable the formation of new memories.

Protein synthesis inhibition and memory: formation vs amnesia

Studies using protein synthesis inhibitors have provided key support for the prevalent view that memory formation requires the initiation of protein synthesis as a primary element of the molecular biology of memory. However, many other interpretations of the amnesia data have received far less attention. These include: (a) protein synthesis may play a constitutive role in memory formation, providing proteins prior to an experience that can be activated by training; (b) protein synthesis may be needed to replace proteins available prior to learning but 'consumed' by learning; (c) inhibition of protein synthesis impairs the well-being of neurons, leading to an inability to deliver resources needed for memory formation; and (d) inhibition of protein synthesis results in abnormal neural functions that interfere with memory. One of these, abnormal release of neurotransmitters after inhibition of protein synthesis, is detailed here, along with a review of many circumstances in which it appears that protein synthesis at the time of training is not required for the formation of new memories. Evidence of activation of cell signaling molecules and transcription factors is another form of support for a role of training-initiated protein synthesis in memory. However, recent findings suggest that many of these molecules are activated by training and remain activated for days after training, i.e. activated for times well beyond those typically invoked for memory consolidation processes. Reviewing these results, this paper suggests that the long-lasting molecular changes may be the basis of a form of intracellular memory, one responsible for up-regulating the probability that a neuron, once activated in this manner, will engage in future plasticity. This view melds ideas of modulation of memory with those of consolidation of memory.

Protein synthesis inhibitors, gene superinduction and memory: too little or too much protein?

To date, the effects of protein synthesis inhibitors (PSI) in learning and memory processes have been attributed to translational arrest and consequent inhibition of de novo protein synthesis. Here we argue that amnesia produced by PSI can be the direct result of their abnormal induction of mRNA-a process termed gene superinduction. This action exerted by PSI involves an abundant and prolonged accumulation of mRNA transcripts of genes that are normally transiently induced. We summarize experimental evidence for the multiple mechanisms and signaling pathways mediating gene superinduction and consider its relevance for PSI-induced amnesia. This mechanistic alternative to protein synthesis inhibition is compared to models of electroconvulsive seizures and fragilexsyndrome associated with enhanced mRNA/protein levels and cognitive deficits.

Amnesia produced by altered release of neurotransmitters after intraamygdala injections of a protein synthesis inhibitor

Amnesia produced by protein synthesis inhibitors such as anisomycin provides major support for the prevalent view that the formation of long-lasting memories requires de novo protein synthesis. However, inhibition of protein synthesis might disrupt other neural functions to interfere with memory formation. Intraamygdala injections of anisomycin before inhibitory avoidance training impaired memory in rats tested 48 h later. Release of norepinephrine (NE), dopamine (DA), and serotonin, measured at the site of anisomycin infusions, increased quickly by approximately 1,000-17,000%, far above the levels seen under normal conditions. NE and DA release later decreased far below baseline for several hours before recovering at 48 h. Intraamygdala injections of a beta-adrenergic receptor antagonist or agonist, each timed to blunt effects of increases and decreases in NE release after anisomycin, attenuated anisomycin-induced amnesia. In addition, similar to the effects on memory seen with anisomycin, intraamygdala injections of a high dose of NE before training impaired memory tested at 48 h after training. These findings suggest that altered release of neurotransmitters may mediate amnesia produced by anisomycin and, further, raise important questions about the empirical bases for many molecular theories of memory formation.

Reconsolidation of a long-term spatial memory is impaired by cycloheximide when reactivated with a contextual latent learning trial in male and female rats

Reconsolidation of long-term memory has become a topic of great interest in recent years, and has the potential to provide important information regarding memory processes and the treatment of memory-related disorders. The present study examined the role of systemic protein synthesis inhibition in reconsolidation of a long-term spatial memory reactivated by a contextual latent learning trial in male and female rats. Using the Morris water maze, we demonstrate that: 1) a contextual latent reactivation treatment enhances memory, 2) systemic protein synthesis inhibition selectively impairs test performance when administered in conjunction with a memory reactivation treatment, and 3) that these effects are more pronounced in female rats. These findings indicate a role for protein synthesis in the reconsolidation of a contextually reactivated long-term spatial memory using the water maze, and a potential differential effect of sex in this apparatus. The role of the strength of the memory trace is discussed and the relevance of these findings to theories of reconsolidation and therapeutic treatment of post-traumatic stress disorder is discussed.

The many faces of amnesia

Results from studies of retrograde amnesia provide much of the evidence for theories of memory consolidation. Retrograde amnesia gradients are often interpreted as revealing the time needed for the formation of long-term memories. The rapid forgetting observed after many amnestic treatments, including protein synthesis inhibitors, and the parallel decay seen in long-term potentiation experiments are presumed to reveal the duration of short-term memory processing. However, there is clear and consistent evidence that the time courses obtained in these amnesia experiments are highly variable within and across experiments and treatments. The evidence is inconsistent with identification of basic temporal properties of memory consolidation. Alternative views include modulation of memory and emphasize the roles that hormones and neurotransmitters have in regulating memory formation. Of related interest, converging lines of evidence suggest that inhibitors of protein synthesis and of other biochemical processes act on modulators of memory formation rather than on mechanisms of memory formation. Based on these findings, memory consolidation and reconsolidation studies might better be identified as memory modulation and "remodulation" studies. Beyond a missing and perhaps unattainable time constant of memory consolidation, some current views of memory consolidation assume that memories, once formed, are generally unmodifiable. It is this perspective that appears to have led to the recent interest in memory reconsolidation. But the view adopted here is that memories are continually malleable, being updated by new experiences and, at the same time, altering the memories of later experiences. Studies of memory remodulation offer promise of understanding the neurobiological bases by which new memories are altered by prior experiences and by which old memories are altered by new experiences.

Cocaine-but not methamphetamine-associated memory requires de novo protein synthesis

Context-induced drug craving and continuous drug use manifest the critical roles of specific memory episodes associated with the drug use experiences. Drug-induced conditioned place preference (CPP) in C57BL/6J mouse model, in this regard, is an appropriate behavioral paradigm to study such drug use-associated memories. Requirement of protein synthesis in various forms of long-term memory formation and storage has been phylogenetically demonstrated. This study was undertaken to study the requirement of protein synthesis in the learning and memory aspect of the conditioned place preference induced by cocaine and methamphetamine, two abused drugs of choice in local area. Since pCREB has been documented as a candidate substrate for mediating the drug-induced neuroadaptation, the pCREB level in hippocampus, nucleus accumbens, and prefrontal cortex was examined for its potential participation in the formation of CPP caused by these psychostimulants. We found that cocaine (2.5 and 5.0 mg/kg/dose)-induced CPP was abolished by the pretreatment of anisomycin (50 mg/kg/dose), a protein synthesis inhibitor, whereas methamphetamine (0.5 or 1.0 mg/kg/dose)-induced CPP was not affected by the anisomycin pretreatment. Likewise, cocaine-induced CPP was mitigated by another protein synthesis inhibitor, cycloheximide (15 mg/kg/injection) pretreatment, whereas methamphetamine-induced CPP remained intact by such pretreatment. Moreover, anisomycin treatment 2h after each drug-place pairing disrupted the cocaine-induced CPP, whereas the same treatment did not affect methamphetamine-induced CPP. An increase of accumbal pCREB level was found to associate with the learning phase of cocaine, but not with the learning phase of methamphetamine. We further found that intraaccumbal CREB antisense oligodeoxynucleotide infusion diminished cocaine-induced CPP, whereas did not affect the methamphetamine-induced CPP. Taken together, these data suggest that protein synthesis and accumbal CREB phosphorylation are essential for the learning and consolidation of the cocaine-induced CPP, whereas methamphetamine-induced CPP may be unrelated to the synthesis of new proteins.

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.

Consolidation of CS and US representations in associative fear conditioning

Much attention has been paid to the associative processes that are necessary to fuse together representations of the various components of an episodic memory. In the present study, we focus on the processes involved in the formation of lasting representations of the individual components that make up a fear-conditioning episode. In one-trial contextual fear conditioning experiments, weak conditioning to context occurs if the shock is delivered immediately following placement of the animal in a novel conditioning apparatus, a phenomenon known as the immediate shock deficit. We show that the immediate shock deficit in mice may be alleviated by pre-exposure to either the context or shock. In using this approach to temporally dissect a contextual fear-conditioning task into its constituent representational and associative processes, we are able to examine directly the processes that are important for formation of lasting representations of the context conditioned stimulus (CS) or unconditioned stimulus (US). Our data indicate that the formation of a lasting representation of the context or shock engages protein synthesis-dependent processes. Furthermore, genetic disruption of cAMP-responsive element binding protein (CREB), a transcription factor that regulates the synthesis of new proteins required for long-term memory, disrupts the formation of lasting context memories. We go on to show that the stress hormone epinephrine modulates the consolidation of a context memory, and reverses consolidation deficits in the CREB-deficient mice. Finally we show that disrupting either NMDA or calcium/calmodulin-dependent kinase II (CaMKII) function impairs consolidation of context memories. Together, these data suggest that this approach is particularly suited for the characterization of molecular and cellular processes underlying the formation of stimulus representations.

Chemoinformatics methods for systematic comparison of molecules from natural and synthetic sources and design of hybrid libraries

Until recently, the field of diversity and library design has more or less ignored natural products as a compound source. This is probably due to at least two reasons. First, combinatorial and reaction-based approaches have been major focal points in the early days of computational library design. In addition, a widespread view is that natural products are often highly complex and not amenable to medicinal chemistry efforts. This contribution introduces recent computational approaches to systematically analyze natural molecules and bridge the gap between natural products and synthetic chemistry programs. Large scale comparisons of natural and synthetic molecules are discussed as well as studies designed to identify 'synthetic mimics' of natural products with specific activity. In addition, a concept for the design of natural/synthetic hybrid libraries is introduced. Although research in this area is still in its early stages, an important lesson to be learned from computational analyses is that there is no need to a priori 'shy away' from natural products as a source for molecular design.

Chemoinformatics methods for systematic comparison of molecules from natural and synthetic sources and design of hybrid libraries

Until recently, the field of diversity and library design has more or less ignored natural products as a compound source. This is probably due to at least two reasons. First, combinatorial and reaction-based approaches have been major focal points in the early days of computational library design. In addition, a widespread view is that natural products are often highly complex and not amenable to medicinal chemistry efforts. This contribution introduces recent computational approaches to systematically analyze natural molecules and bridge the gap between natural products and synthetic chemistry programs. Large scale comparisons of natural and synthetic molecules are discussed as well as studies designed to identify 'synthetic mimics' of natural products with specific activity. In addition, a concept for the design of natural/synthetic hybrid libraries is introduced. Although research in this area is still in its early stages, an important lesson to be learned from computational analyses is that there is no need to a priori 'shy away' from natural products as a source for molecular design.

Amnesia induced by stimulation of the amygdala is attenuated by hexamethonium

Bilateral subseizure stimulation of the amygdala given immediately following training in an inhibitory avoidance task produced retrograde amnesia. Hexamethonium (3.0 and 10.0 mg/kg), a peripherally acting nicotinic cholinergic antagonist, attenuated the retention deficits induced by amygdala stimulation if the drug was given 30 min prior to, but not immediately following training. Hexamethonium had no effect in normal unoperated animals, but did produce a retention deficit in operated control (nonstimulated) animals if it was given immediately following training (3.0 and 10.0 mg/kg). The results suggest that memory deficits induced by electrical stimulation of the amygdala are associated with, or perhaps mediated in some way by peripheral autonomic function.

Effects of a single administration of oxytocin or vasopressin and their interactions with two selective receptor antagonists on memory storage in mice

The neuropeptides arginine vasopressin (AVP) and oxytocin (OT) have been thought to play a significant role in behavioral regulation in general and in learning and memory in particular. Experimental evidence suggests that AVP improves, and OT impairs, learning and memory. The present paper investigates the posttraining effects of OT and of an OT receptor antagonist, and their interaction, on memory storage in mice. Additional studies were conducted to determine the specificity of the interaction between OT and its receptors. Male Swiss mice were tested 48 h after training in a one-trial step-through inhibitory avoidance task. Immediate posttraining subcutaneous injection of OT (0.01, 0.03, 0.10, 0.30, and 1.00 microg/kg) impaired retention performance. The dose-response curve showed a U-shaped form, with a significant impairment seen at doses of 0.10 and 0.30 microg/kg of OT. In contrast, the immediate posttraining administration of the putative oxytocin receptor antagonist d(CH2)5[Tyr(Me)2, Thr4, Thy-NH(9)2]OVT (AOT, 0.03, 0.10, 0.30, and 1.00 microg/kg) significantly enhanced retention performance. The dose-response curve was an inverted "U" in this range of doses. However, of the doses tested, only 0.30 microg/kg was effective. Neither OT nor AOT affected response latencies in mice not given the footshock on the training trial, indicating that the actions of both treatments on retention performance were not due to nonspecific proactive effects on response latencies. Neither the imparing effects of OT (0.10 microg/kg) nor the enhancing effects of AOT (0.30 microg/kg) were seen when the training-treatment interval was 180 min, suggesting that both treatments influenced the storage of recently acquired information. The effects of OT (0.10 microg/kg) on retention were prevented by AOT (0.03 microg/kg) administered immediately after training, but 10 min prior to oxytocin treatment. This dose of antagonist did not affect retention by itself, either under the standard experimental conditions or in mice trained with a lower level of footshock. On the contrary, OT (0.10 microg/kg) impaired retention in mice pretreated with the V1a vasopressin receptor antagonist d(CH2)5[Tyr(Me)2]AVP (0.01 microg/kg), which, however, was able to prevent the enhancement of retention induced by posttraining administration of AVP (0.03 microg/kg). Finally, the effects of AVP (0.03 microg/kg) on retention were not prevented by AOT (0.03 microg/kg). Considered together, these findings suggest that the impairment of retention of an inhibitory avoidance response in mice induced by posttraining oxytocin is probably due to an interaction of the neuropeptide with specific receptors.

TRH mimetics: differentiation of antiamnesic potency from antidepressant effect

For the purpose of rational modification of the TRH molecule, we were pursuing an approach that consists of two steps: (1) 'obligatory' replacement of histidine with glutamine in TRH and (2) the application of conformational constraints for putative bioactive conformation I stabilized by an intramolecular hydrogen bond between C-terminal carboxamide proton and alpha-carbonyl of histidyl (glutaminyl), and conformation II formed by an intramolecular hydrogen bond between alpha-carbonyl of pyroglutamyl and prolinamide proton. Significant antiamnesic potency was discovered in the passive avoidance test (ECS and Scopolamine induced amnesia) for conformation II mimic (8S,10aS)-8-carbamoyl-1,2,3,6,7,8,9,10a- octahydro-5H,10H-pyrrolo[1,2-a][1,4]diazocin-5,10-dione (2) at doses of 0.1 and 1.0 mg/kg. EEG analysis indicates a mild activating effect of compound 2 on EEG, which is similar to that of piracetam and differs from hard amphetamine activation. Conformation I mimic 3-(2-carbamoylethyl)-2,3,6,7,8,8a-hexahydro-1H,4H-pyrrolo[1,2-a] pyrazin-1,4-dione (1) exhibited an antidepressant effect at a dose of 1 mg/kg. The transition from two putative quasi-cyclic bioactive conformations of TRH and its obligatory similar analogue [Gln2]-TRH to their cyclic mimics led to differentiation of antiamnesic and antidepressant activity of TRH.

Amphetamine enhances human-memory consolidation

Although it is generally accepted that CNS stimulants have enhancing effects on long-term storage processes in laboratory animals, little is known about their influence on human learning. We report a series of experiments with free recall of lists of unrelated words, demonstrating a significant enhancement on long-term retention after amphetamine administration. A gradual increase of recall was observed up to 1 h after learning, remaining stable for at least 3 days, after oral administration before learning as well as intramuscular injection after learning. The results show that research on humans with drug-induced memory-enhancement techniques is necessary to supplement the animal studies for the understanding of the mechanisms involved in information consolidation.

The direction of the conditioned natural killer cell response can be re-directed with indomethacin and/or handling

We have used the pairing of camphor odor conditioned stimulus (CS) and injection of poly I:C unconditioned stimulus (US) in a short 3 day single trial conditioning paradigm. Conditioning was done by exposing mice to the CS/US combination on day 0 and reexposing the conditioned animals to the CS on day 2. This results in a conditioned augmentation of the natural killer (NK) cell response. Indomethacin treatment and/or handling stress induced by simply measuring rectal temperature was found to dramatically alter the direction of the conditioned NK cell response. Conditioning of indomethacin treated mice produced a conditioned suppression of the NK cell response mimicking a conditioned tolerance response. If handling stress was superimposed on day 2 the conditioned suppression response was replaced by a conditioned augmentation of the NK cell response. Even with one trial conditioning, drugs and handling stress can serve as additional cues to alter the direction of the conditioned response. The studies also show that the conditioning of the fever response is independent of conditioning of the NK cell response.

Behavioral screening for cognition enhancers: from indiscriminate to valid testing: Part I

Preclinical efforts to detect and characterize potential cognition enhancers appear to have been dominated by a strategy of demonstrating a wide variety of apparently beneficial behavioral effects with little attention given to the specific psychological mechanisms underlying behavioral enhancement. In particular, the question of whether or not behavioral facilitation is based on relevant mnemonic mechanisms and is independent of the stimulus properties and/or the motivational and attentional components of a task is not often considered. As a result, an overwhelming number of compounds have failed to produce the clinical effects predicted for them on the basis of preclinical research. The available data suggest that a more successful approach requires deductive research strategies rather than the indiscriminate accumulation of apparently beneficial effects in a variety of behavioral tasks and animal models. The first step towards such an approach is a systematic and rigorous evaluation of the different aspects of validity for the models most frequently used in preclinical research. It is concluded that a combination of good construct validity and good face validity represents a necessary condition for screening tests with predictive validity, and that the most popular paradigms fail to fulfil these criteria. Future screening programs for cognition enhancers will probably be characterized by a depreciation of "fast and dirty tests" in favor of approaches focussing on the validity of the effects of potential cognition enhancers.

The effects of preexposure to the drug on state dependent retention

The present experiment examined whether previous experience with a drug would decrease the potential of the drug to produce state dependent retention (SDR) for a passive avoidance response in rats. In the first experiment, a single injection of sodium pentobarbital (20 mg/kg) given on six consecutive days before the training day slightly reduced, but did not block, pentobarbital-induced SDR. In Experiment Two, four preexposure injections of 5 IU/kg insulin reduced the magnitude of memory loss produced by administration of the hormone prior to training. As with pentobarbital, however, preexposure to insulin did not completely block the amnestic effect of the hormone. A subsequent experiment demonstrated that the decrease in the strength of insulin-induced SDR in insulin preexposed rats was not the result of enhanced acquisition. Collectively, these data indicate that noncontingent preexposure to an amnestic treatment may decrease the magnitude of memory loss that would normally result from the administration of that treatment during training.

The timing of an injection procedure affects pharmacological actions on memory

In a series of experiments examining the effects of the protein synthesis inhibitor anisomycin on memory for a novel active avoidance task in mice, we found that the timing of administering the drug (pretraining or posttraining) affected its amnestic potency. Anisomycin injected after training was more effective than when injected before training. Adding a saline injection, such that all groups received both pre- and posttraining injections, resulted in greater amnesia with anisomycin given before rather than after training. These results indicate that the procedure of drug administration alters the effectiveness of amnestic agents.

Effect of reserpine on retention of the conditioned NK cell response

The effect of reserpine and 6-hydroxydopamine on the learned conditioned natural killer (NK) cell response was investigated in mice. Reserpine given at 2.5 mg/kg, 24 hr prior to reexposure to camphor-conditioned stimulus on days 6 and 8 blocked the recall of conditioned NK cell response to a significant extent. In other words, the NK cell activity of conditioned mice, treated with reserpine and reexposed to the conditioned stimulus, was similar to the nonconditioned (NC) group. A conditioned increase in NK cell response was still evident in mice treated with 6-OHDA.

Effects of a new TRH analogue, YM-14673, on disturbance of passive avoidance learning in senescence-accelerated mice

Effects of a new TRH analogue, YM-14673 (N alpha-[[(S)-4-oxo-2-azetidinyl]carbonyl]-L-histidyl-L-prolinamide dihydrate), on disturbance of passive avoidance behavior were observed in senescence-accelerated mice (SAM). Latency of step-through in SAM-P/8/Ta (SAM-P/8, senescence-prone substrain) was significantly shorter than that in SAM-R/1/Ta (SAM-R/1, senescence-resistant substrain). Successive oral administration of YM-14673 (1 and 10 mg/kg) and TRH (10 mg/kg) for 3 weeks prolonged the shortened latency of step-through. These results suggest that YM-14673 is more potent than TRH in antiamnesic activities.

Anisomycin and amnesia in the chick: state-dependent effects are not present with intracranial injections

It has recently been suggested that intraperitoneal (IP) injection of anisomycin (ANI) in the chick produces amnesia for a one-trial passive avoidance task in a state-dependent manner. We have examined the behavioral and biochemical effects of IP and intracranial (IC) injections of ANI in chicks trained on a one-trial passive avoidance task. IC injection of ANI produced 35% brain protein synthesis inhibition whereas IP injection produced only negligible amounts of protein synthesis inhibition in the brain. IC injection of ANI produced amnesia and was not state-dependent. Patterns of behavior consistent with state-dependent effects were produced by IP injection of ANI. These experiments indicate that there are differences in the pattern of results produced by IP and IC injection of ANI and support the hypothesis that the expression of long-term memory in chicks is associated with protein synthesis.

An hypothesis on the role of glucose in the mechanism of action of cognitive enhancers

This review presents evidence that some cognition enhancing drugs produce their beneficial effects on learning and memory by increasing the availability of glucose for uptake and utilization into the brain. The hypothesis further suggests that many cognition enhancing drugs act through a peripheral mechanism rather than directly on the brain. The general hypothesis is supported by four independent and converging pieces of evidence: 1) Some cognition enhancing drugs may not cross the blood-brain barrier, but can still facilitate memory; 2) Some cognition enhancing drugs are effective only when injected peripherally, but not when injected directly into the brain; 3) Many cognition enhancing drugs are not effective after adrenalectomy; 4) Cognitive function is correlated with glucose regulation in aged animals and humans. These four lines of research have implications for the role of glucose in the action of specific cognitive enhancers.

Specificity of piracetam's anti-amnesic activity in three models of amnesia in the mouse

The effects of piracetam on the amnesias induced by scopolamine, diazepam and electroconvulsive shock (ECS) were studied in a passive avoidance procedure in the mouse and compared with the interactions of piracetam with the major behavioral effects of these treatments, namely scopolamine-induced hyperactivity, diazepam-induced release of punished behavior (Four Plates Test) and ECS-induced convulsions. Amnesia was induced by injecting scopolamine or diazepam (1 mg/kg, IP) 30 minutes before or applying ECS immediately after the first session (S1) of the passive avoidance task. Piracetam was studied at 3 doses (512, 1024 and 2048 mg/kg) administered PO 60 minutes before S1. Retention was measured 24 hours later (S2) in the absence of any treatment. Piracetam dose-dependently attenuated the memory deficits induced by the three amnesic treatments but did not affect either scopolamine-induced hyperactivity, diazepam-induced release of punished behavior or ECS-induced convulsions. These results point to the specificity of piracetam's anti-amnesic activity and, in particular, suggest that piracetam can suppress the memory disturbances induced by diazepam without affecting diazepam's anxiolytic activity. The test battery employed would therefore seem highly suitable for evaluating the potential nootropic activity of novel compounds.

Effects of the nicotinic receptor blocker mecamylamine on radial-arm maze performance in rats

Lesions of cholinergic neurons have been found by many investigators to impair choice accuracy in the radial arm maze. Because muscarinic receptor blockers, such as scopolamine, have also repeatedly been found to impair choice accuracy in the radial-arm maze, it has generally been thought that the critical effect of cholinergic lesions is the deafferentation of muscarinic receptors. The possible involvement of nicotinic receptors in the cholinergic bases of cognitive performance in the radial-arm maze has not been as well investigated. The present study examined the effects of the blockade of nicotinic receptors on performance of female Sprague-Dawley rats in the radial-arm maze. Acute administration of the the nicotinic receptor blocker, mecamylamine (10 mg/kg) was found to significantly impair radial-arm maze choice accuracy. This dose also caused a significant increase in response latency in the maze. The effect on choice behavior but not locomotor speed seemed to be due to the central effects of mecamylamine, because administration of the peripheral nicotine receptor blocker, hexamethonium (20 mg/kg), did not impair choice accuracy, even though it did increase response latency to a similar degree as the 10-mg/kg dose of mecamylamine. Lower doses of mecamylamine (2.5 and 5 mg/kg) did not impair choice accuracy. These results indicate that central nicotinic as well as muscarinic cholinergic receptors are involved with cognitive functioning.

The impairment of retention induced by pentylenetetrazol in mice may be mediated by a release of opioid peptides in the brain

Pentylenetetrazol (PTZ, 45 mg/kg, ip) impaired retention of a one-trial step-through inhibitory avoidance task when injected into male Swiss mice 10 min after training, as indicated by retention performance 48 h later. The amnestic effect of PTZ was prevented by naltrexone (0.01 or 0.10 mg/kg, ip) administered after training, but prior to PTZ-treatment. On the contrary, neither naltrexone methyl bromide (0.01, 0.10, or 10.0 mg/kg, ip), a quaternarium analog of naltrexone, nor MR2266 (0.01 or 0.10 mg/kg, ip), a putative kappa opiate receptor antagonist, modified the behavioral effects of PTZ. On the other hand, the body seizures produced by PTZ were unaffected by any of the three opiate receptor antagonists that were given before the convulsant. Taken together, these results suggest that the effects of PTZ on retention are mediated, at least in part, by opioid peptides of central origin, and rules out a possible participation of opioid peptides derived from prodynorphin-precursor molecule. Administration of beta-endorphin (0.01 or 0.10 microgram/kg, ip) 10 min prior to testing attenuate the retrograde amnesia caused by PTZ. The effect of beta-endorphin was prevented by the simultaneous administration of naltrexone (0.10 mg/kg, ip) prior to testing. Naltrexone has no effect of its own upon retrieval. These results suggest that the impairment of retention induced by PTZ is probably due, at least in part, to a release of opioid peptides in the brain during the post-training period. PTZ given after training do not affect consolidation or memory storage, as mice thus treated may retrieve the learned information when they are submitted to an appropriate neurohumoral and/or hormonal state in the test session, that is, beta-endorphin injection. Therefore, the action of PTZ would be primarily at the level of the mechanism that make stored information available for late retrieval.

Effects of dehydroepiandrosterone and its sulfate on brain tissue in culture and on memory in mice

Low concentrations of dehydroepiandrosterone (DHEA) and dehydroepiandrosterone sulfate (DHEAS) enhanced neuronal and glial survival and/or differentiation in dissociated cultures of 14-day mouse embryo brain. Posttrial intracisternal injection into the brains of mice undergoing active avoidance training alleviated amnesia and enhanced long-term memory. By minimizing degenerative changes in injured nerve tissue and facilitating plastic changes, DHEA and DHEAS may be of use in treatment of neurodegenerative and memory disorders in man.

The neuropathology of amnesia

Relations between brain damage and memory disturbance are outlined with emphasis on the so-called amnesic syndrome. Following a brief introduction into forms of memory and memory failures, the basic causes of brain damaage (with relevance to amnestic failures) are described. Thereafter, the two best-known forms of brain damage-amnesia relations are reviewed: the consequences of damage to medial temporal lobe structures and to diencephalic regions. For the cases with medial temporal lobe damage, evidence is reported in greater detail for H.M., who has been examined more than any other amnesic patient for more than 30 years now, as a considerable amount of literature has accumulated on his behavior in diverse situations. Other cases with more or less circumscribed damage to medial temporal lobe structures are reviewed so as to outline criteria for or against the hypothesis that there are regions within the medial temporal lobe whose damage might be critical for the amnesic syndrome. Two cases of diencephalic amnesia are summarized in particular (cases of Mair et al., 1979) as they have received extensive neuropsychological and neuropathological investigation. Other cases with, for example, Korsakoff's disease are reviewed, as well as cases with diencephalic, or combined mesencephalic-diencephalic damage without nutritional causes. A third group of patients with massive, but still selective amnesic disturbances are then described: cases of basal forebrain damage, followed by descriptions of Alzheimer's disease which has similarities in the underlying neuropathology. This leads over to cases with more generalized intellectual deteriorations (dementia), which may have developed on the basis of primarily cortical damage or damage principally to basal ganglia structures. After reviewing cases with mainly material-specific memory failures--usually as a consequence of restricted neocortical damage--a separate section follows on patients in whom retrograde amnesia is the prominent symptom. The contribution of animal models of human amnesia is critically reviewed and discrepancies are analyzed between human and animal memory disturbances. This section emphasizes the value of investigating inter-dependencies between brain structures by pointing out that relations between memory disturbances and brain damage may be more complicated than apparent from a simple structure-function assignment. This aspect is further followed up in the conclusions.

Naloxone attenuates amnesia caused by amygdaloid stimulation: the involvement of a central opioid system

This study investigated the effect of naloxone on amnesia produced by subseizure amygdaloid stimulation. Animals were trained in an inhibitory avoidance task, and given amygdaloid stimulation following training. Immediately after training, prior to stimulation, naloxone was injected either peripherally (i.p.) or into the bed nucleus of the stria terminalis (BNST) where the Met-enkephalin-containing fibers from the amygdala terminate. Amygdaloid stimulation caused retention deficits. The deficits were attenuated by 3.0 mg/kg naloxone given peripherally or by 1.0 microgram or 0.3 microgram naloxone injected bilaterally into the BNST. The attenuative effect was anatomically and receptor specific: 0.3 microgram of naloxone injected into the caudate nucleus was ineffective; the attenuative effect of naloxone was antagonized by simultaneous injection of 1.5 or 4.5 micrograms levorphanol into the BNST. These results suggest that endogenous opioids, possibly the enkephalins of the stria terminalis released into the BNST following amygdaloid stimulation, are at least partially involved in mediating the effect of amygdaloid stimulation on memory.