Recalling an aversive experience by day-old chicks is not dependent on somatic protein synthesis

Microtubules as Regulators of Neural Network Shape and Function: Focus on Excitability, Plasticity and Memory

Neuronal microtubules (MTs) are complex cytoskeletal protein arrays that undergo activity-dependent changes in their structure and function as a response to physiological demands throughout the lifespan of neurons. Many factors shape the allostatic dynamics of MTs and tubulin dimers in the cytosolic microenvironment, such as protein–protein interactions and activity-dependent shifts in these interactions that are responsible for their plastic capabilities. Recently, several findings have reinforced the role of MTs in behavioral and cognitive processes in normal and pathological conditions. In this review, we summarize the bidirectional relationships between MTs dynamics, neuronal processes, and brain and behavioral states. The outcomes of manipulating the dynamicity of MTs by genetic or pharmacological approaches on neuronal morphology, intrinsic and synaptic excitability, the state of the network, and behaviors are heterogeneous. We discuss the critical position of MTs as responders and adaptative elements of basic neuronal function whose impact on brain function is not fully understood, and we highlight the dilemma of artificially modulating MT dynamics for therapeutic purposes.

Impairing memory reconsolidation with propranolol in healthy and clinical samples: a meta-analysis

BACKGROUND

Reconsolidation impairment using propranolol is a novel intervention for mental disorders with an emotional memory at their core. In this systematic review and meta-analysis, we examined the evidence for this intervention in healthy and clinical adult samples.

METHODS

We searched 8 databases for randomized, double-blind studies that involved at least 1 propranolol group and 1 placebo group. We conducted a meta-analysis of 14 studies (n = 478) in healthy adults and 12 studies in clinical samples (n = 446).

RESULTS

Compared to placebo, reconsolidation impairment under propranolol resulted in reduced recall of aversive material and cue-elicited conditioned emotional responses in healthy adults, as evidenced by an effect size (Hedges g) of -0.51 (p = 0.002, 2-tailed). Moreover, compared to placebo, reconsolidation impairment under propranolol alleviated psychiatric symptoms and reduced cue-elicited reactivity in clinical samples with posttraumatic stress disorder, addiction or phobia (g = -0.42, p = 0.010).

LIMITATIONS

Methodological differences between studies posed an obstacle for identifying sources of heterogeneity.

CONCLUSION

Reconsolidation impairment is a robust, well-replicated phenomenon in humans. Its clinical use is promising and deserves further controlled investigation.

Microtubule ionic conduction and its implications for higher cognitive functions

The neuronal cytoskeleton has been hypothesized to play a role in higher cognitive functions including learning, memory and consciousness. Experimental evidence suggests that both microtubules and actin filaments act as biological electrical wires that can transmit and amplify electric signals via the flow of condensed ion clouds. The potential transmission of electrical signals via the cytoskeleton is of extreme importance to the electrical activity of neurons in general. In this regard, the unique structure, geometry and electrostatics of microtubules are discussed with the expected impact on their specific functions within the neuron. Electric circuit models of ionic flow along microtubules are discussed in the context of experimental data, and the specific importance of both the tubulin C-terminal tail regions, and the nano-pore openings lining the microtubule wall is elucidated. Overall, these recent results suggest that ions, condensed around the surface of the major filaments of the cytoskeleton, flow along and through microtubules in the presence of potential differences, thus acting as transmission lines propagating intracellular signals in a given cell. The significance of this conductance to the functioning of the electrically active neuron, and to higher cognitive function is also discussed.

Neural cytoskeleton capabilities for learning and memory

This paper proposes a physical model involving the key structures within the neural cytoskeleton as major players in molecular-level processing of information required for learning and memory storage. In particular, actin filaments and microtubules are macromolecules having highly charged surfaces that enable them to conduct electric signals. The biophysical properties of these filaments relevant to the conduction of ionic current include a condensation of counterions on the filament surface and a nonlinear complex physical structure conducive to the generation of modulated waves. Cytoskeletal filaments are often directly connected with both ionotropic and metabotropic types of membrane-embedded receptors, thereby linking synaptic inputs to intracellular functions. Possible roles for cable-like, conductive filaments in neurons include intracellular information processing, regulating developmental plasticity, and mediating transport. The cytoskeletal proteins form a complex network capable of emergent information processing, and they stand to intervene between inputs to and outputs from neurons. In this manner, the cytoskeletal matrix is proposed to work with neuronal membrane and its intrinsic components (e.g., ion channels, scaffolding proteins, and adaptor proteins), especially at sites of synaptic contacts and spines. An information processing model based on cytoskeletal networks is proposed that may underlie certain types of learning and memory.

Plastic synaptic networks of the amygdala for the acquisition, expression, and extinction of conditioned fear

The last 10 years have witnessed a surge of interest for the mechanisms underlying the acquisition and extinction of classically conditioned fear responses. In part, this results from the realization that abnormalities in fear learning mechanisms likely participate in the development and/or maintenance of human anxiety disorders. The simplicity and robustness of this learning paradigm, coupled with the fact that the underlying circuitry is evolutionarily well conserved, make it an ideal model to study the basic biology of memory and identify genetic factors and neuronal systems that regulate the normal and pathological expressions of learned fear. Critical advances have been made in determining how modified neuronal functions upon fear acquisition become stabilized during fear memory consolidation and how these processes are controlled in the course of fear memory extinction. With these advances came the realization that activity in remote neuronal networks must be coordinated for these events to take place. In this paper, we review these mechanisms of coordinated network activity and the molecular cascades leading to enduring fear memory, and allowing for their extinction. We will focus on Pavlovian fear conditioning as a model and the amygdala as a key component for the acquisition and extinction of fear responses.

Remembering that things have changed: a review of the cellular mechanisms of memory re-consolidation in the day-old chick

It has been one of the unshakeable orthodoxies of memory research that memory is initially laid down in a labile form for a short period following the experience and that over time the memory is "fixed" or "consolidated" into the physical structure of the brain. Over the last decade a large body of data has gathered which demonstrates that a "consolidated" memory can be returned to a labile state following retrieval of material from the store, which can then be re-consolidated, incorporating the newly acquired information into the representation of the world. The process of re-consolidation thus provides a sensible means for the crucial process of memory updating to occur. The paper focuses on pharmaco-behavioural experiments undertaken in our laboratories as well as in those of other groups which use the day-old chick as subject and the passive avoidance learning (PAL) task to examine the behavioural and metabolic parameters of re-consolidation. The data indicate that the consolidation and the re-consolidation processes are similar but not identical physiological processes. The re-processing of the memory following a re-consolidation involves each of the glutamatergic, adrenergic and dopaminergic neurotransmitter systems as well as re-activation of protein synthesis associated with the respective traces. In the chick model system, the ability to undertake re-consolidation is transient, and is observed only for a maximum of 24-48 h following the initial training event. Controversy persists as to whether the re-consolidated memory represents a new memory or whether it is a modification of the original memory processing.

Aposematic colouration enhances memory formation in domestic chicks trained in a weak passive avoidance learning paradigm

The one-trial passive avoidance learning task is commonly used in avian research to explore anatomical, cellular and molecular parameters of learning and memory. Many factors are known to influence the effectiveness and/or duration of such learning events. Combinations of novel odours, such as pyrazine, and aposematic colours, such as brig ht yellow or red, have been shown to induce a long-lasting aversion to food crumbs in 'visual' predators, including birds such as the domestic chick (1). The aim of this study was to (a) examine whether visual complexity played a role in the generation of an aversive response to a novel visual stimulus and (b) to establish whether the duration of memory of an aversive experience could be modified by altering the visual properties of the stimulus. In the first experiment, naive domestic chicks were trained on a weakly aversive one-trial passive avoidance bead task, in which chicks were allowed to peck at a bead coated with a 10% solution of the bitter-tasting and odorous substance methylanthranilate (MeA). The chicks were trained with (allowed to peck) one of four differently coloured beads dipped in 10% MeA. Chrome, black, yellow or black-and-yellow striped beads were used. 'Recall' of the aversive bead was examined by presenting the (clean) training bead 24h after training and monitoring avoidance to it compared to a 'neutral' white bead. A high proportion (63%) of chicks trained with the black and yellow striped bead avoided it 24h after training, whereas little or no avoidance was seen in response to chrome, yellow or black beads. In a second experiment naive domestic chicks were all trained once only with a black and yellow striped bead coated in a 10% MeA solution, but this time, were tested 24h later, once only, with either a black, a yellow or a black and yellow striped bead. Nearly 60% of chicks tested with a black and yellow striped bead showed avoidance of the bead, whereas only 23% of those tested with a black bead and 14% tested with a yellow bead showed avoidance. These results confirm the importance of complex warning colouration, when paired with a novel olfactory cue and a bitter taste, in avoidance learning. We conclude that the chicks' response to monochromatic colours (e.g. yellow or black) is not affected by their previous experience with a conspicuously patterned stimulus (yellow and black stripes). Moreover, it suggests a predisposition for chicks to attend to aversive cues associated with 'naturalistic' high contrast colour cue combinations such as black and yellow.

Macromolecular synthesis, distributed synaptic plasticity, and fear conditioning

Recent work from a number of laboratories has provided new and important insights about how gene expression is altered by experience and how these molecular changes may provide a substrate for the long-term storage of new memories. Here, we review a series of recent studies using aversive Pavlovian conditioning in rats as a well characterized model system in which experience-dependent alterations in gene expression can be manipulated and quantified within a specific neural circuit. We highlight some of the issues involved in using broad-spectrum inhibitors of mRNA and protein synthesis to study cellular changes underlying the formation and long-term stability of memory and discuss the idea that these changes occur over widespread, behaviorally-defined, networks of cells. We also discuss the idea that the maintenance of memory and its susceptibly to disruption after retrieval may relate to local protein synthesis in dendrites. Finally, a series of recent experiments from our laboratory studying the role of a specific signaling pathway (mTOR) which regulates translational processes and memory formation in the amygdala and hippocampus during fear conditioning are reviewed.

Effect of gestational ethanol exposure on long-term memory formation in newborn chicks

Fetal alcohol syndrome (FAS), a condition occurring in some children of mothers who have consumed alcohol during pregnancy, is characterized by craniofacial malformations, and physical and mental retardation. It is significant that even children with history of gestational ethanol exposure but relatively unaffected overall IQ performance, often exhibit learning difficulties and behavioral problems, suggestive of impaired memory formation. Hence, the specific aim of this study was to examine memory formation in chicks exposed to ethanol during early gestation toward the understanding of neurobehavioral disturbances in FAS. Chicks were exposed to alcohol on gestational days 1-3 by injection of ethanol into the airspace of freshly fertilized eggs. The effects of prenatal ethanol on physical growth and development, and memory formation were studied. The one-trial passive avoidance learning paradigm in 1-day-old chicks was used to study memory formation in these chicks. It was observed that chick embryos exposed to 10% ethanol on gestational days 1-3 had significant reduction in all body parameters when compared with appropriate controls. Further, ethanol-exposed chick embryos had significantly impaired (P<.05) long-term memory (LTM) formation after training, though short-term or intermediate-term memory formation was unimpaired. Thus, the findings of the current study demonstrate the detrimental effects of ethanol exposure during early pregnancy on developing chick embryos in general and on memory formation in particular. Hence, it is suggested that impairment in LTM could be a fundamental mechanism for learning disorders and neurobehavioral abnormalities observed in FAS.

Translational control via the mammalian target of rapamycin pathway is critical for the formation and stability of long-term fear memory in amygdala neurons

The mammalian target of rapamycin kinase (mTOR) regulates protein synthesis in neurons at the translational level through phosphorylation of several intracellular targets. Recent work in invertebrates indicates that mTOR-dependent translational control may be critical for the induction and maintenance of activity-dependent synaptic plasticity underlying memory formation. Here, we report that training rats in a simple fear conditioning procedure evokes a time-dependent increase in the phosphorylation of p70s6 kinase, a major direct downstream target of mTOR. When the activation of mTOR was prevented by posttraining injection of rapamycin into the amygdala, formation of the memory and the increase in p70s6 kinase phosphorylation was attenuated. Furthermore, when rapamycin was applied to the amygdala after the recall of a previously stored fear memory, subsequent retention was disrupted, indicating that local translational control at active synapses is required for the stability as well as the formation of long-term memory in this system.

Retrieval induces hippocampal-dependent reconsolidation of spatial memory

Nonreinforced retrieval can cause extinction and/or reconsolidation, two processes that affect subsequent retrieval in opposite ways. Using the Morris water maze task we show that, in the rat, repeated nonreinforced expression of spatial memory causes extinction, which is unaffected by inhibition of protein synthesis within the CA1 region of the dorsal hippocampus. However, if the number of nonreinforced retrieval trials is insufficient to induce long-lasting extinction, then a hippocampal protein synthesis-dependent reconsolidation process recovers the original memory. Inhibition of hippocampal protein synthesis after reversal learning sessions impairs retention of the reversed preference and blocks persistence of the original one, suggesting that reversal learning involves reconsolidation rather than extinction of the original memory. Our results suggest the existence of a hippocampal protein synthesis-dependent reconsolidation process that operates to recover or update retrieval-weakened memories from incomplete extinction.

Reconsolidation: the advantage of being refocused

Ample evidence suggests that upon their retrieval, items in long-term memory enter a transient special state, in which they might become prone to change. The process that generates this state is dubbed 'reconsolidation'. The dominant conceptual framework in this revitalized field of memory research focuses on whether reconsolidation resembles consolidation, which is the process that converts an unstable short-term memory trace into a more stable long-term trace. However, this emphasis on the comparison of reconsolidation to consolidation deserves reassessment. Instead, the phenomenon of reconsolidation, irrespective of its relevance to consolidation, provides a unique opportunity to tap into the molecular, cellular and circuit correlates of memory persistence and retrieval, of which we currently know only little.