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

Stress-induced nuclear speckle reorganization is linked to activation of immediate early gene splicing

Current models posit that nuclear speckles (NSs) serve as reservoirs of splicing factors and facilitate posttranscriptional mRNA processing. Here, we discovered that ribotoxic stress induces a profound reorganization of NSs with enhanced recruitment of factors required for splice-site recognition, including the RNA-binding protein TIAR, U1 snRNP proteins and U2-associated factor 65, as well as serine 2 phosphorylated RNA polymerase II. NS reorganization relies on the stress-activated p38 mitogen-activated protein kinase (MAPK) pathway and coincides with splicing activation of both pre-existing and newly synthesized pre-mRNAs. In particular, ribotoxic stress causes targeted excision of retained introns from pre-mRNAs of immediate early genes (IEGs), whose transcription is induced during the stress response. Importantly, enhanced splicing of the IEGs ZFP36 and FOS is accompanied by relocalization of the corresponding nuclear mRNA foci to NSs. Our study reveals NSs as a dynamic compartment that is remodeled under stress conditions, whereby NSs appear to become sites of IEG transcription and efficient cotranscriptional splicing.

Synapse integrity and function: Dependence on protein synthesis and identification of potential failure points

Synaptic integrity and function depend on myriad proteins - labile molecules with finite lifetimes that need to be continually replaced with freshly synthesized copies. Here we describe experiments designed to expose synaptic (and neuronal) properties and functions that are particularly sensitive to disruptions in protein supply, identify proteins lost early upon such disruptions, and uncover potential, yet currently underappreciated failure points. We report here that acute suppressions of protein synthesis are followed within hours by reductions in spontaneous network activity levels, impaired oxidative phosphorylation and mitochondrial function, and, importantly, destabilization and loss of both excitatory and inhibitory postsynaptic specializations. Conversely, gross impairments in presynaptic vesicle recycling occur over longer time scales (days), as does overt cell death. Proteomic analysis identified groups of potentially essential 'early-lost' proteins including regulators of synapse stability, proteins related to bioenergetics, fatty acid and lipid metabolism, and, unexpectedly, numerous proteins involved in Alzheimer's disease pathology and amyloid beta processing. Collectively, these findings point to neuronal excitability, energy supply and synaptic stability as early-occurring failure points under conditions of compromised supply of newly synthesized protein copies.

Spatiotemporally resolved protein synthesis as a molecular framework for memory consolidation

De novo protein synthesis is required for long-term memory consolidation. Dynamic regulation of protein synthesis occurs via a complex interplay of translation factors and modulators. Many components of the protein synthesis machinery have been targeted either pharmacologically or genetically to establish its requirement for memory. The combination of ligand/light-gating and genetic strategies, that is, chemogenetics and optogenetics, has begun to reveal the spatiotemporal resolution of protein synthesis in specific cell types during memory consolidation. This review summarizes current knowledge of the macroscopic and microscopic neural substrates for protein synthesis in memory consolidation. In addition, we highlight future directions for determining the localization and timing of de novo protein synthesis for memory consolidation with tools that permit unprecedented spatiotemporal precision.

Protein synthesis and actin polymerization in the rapid effects of 17β-estradiol on short-term social memory and dendritic spine dynamics in female mice

Estrogens rapidly facilitate learning and memory, including social recognition - the ability of an animal to recognize another. In ovariectomized female mice, systemic or dorsal hippocampal administration of 17β-estradiol (E2) facilitates short-term social recognition memory within 40 min. Within the same timeframe, E2 increases dendritic spine density in CA1 dorsal hippocampal neurons of behavioural task-naïve mice and in hippocampal sections. Mechanisms underlying these effects remain unclear. Estrogens rapidly modulate actin cytoskeletal dynamics through actin polymerization and the translation of key synaptic proteins. We first determined doses of actin polymerization inhibitor latrunculin A (LAT) and protein synthesis inhibitor anisomycin (ANI) that would block short-term social recognition memory when infused into the dorsal hippocampus of ovariectomized female mice 15 min prior to testing. The highest doses that did not block social recognition prevented the facilitating effects of E2, whereas DNA transcription inhibitor, actinomycin D, could not block social recognition. As task performance may interfere with E2-facilitated increases in dendritic spine density, dendritic spine density and length were examined in task-performing and task-naïve mice. E2 increased dendritic spine density 15 but not 40 min following treatment, regardless of whether the animal had performed the social recognition task. This effect was blocked by LAT, but not ANI. Thus, both actin polymerization and protein synthesis are necessary for E2 to rapidly facilitate social recognition, whereas actin polymerization, but not protein synthesis, is required for the rapid increase in dendritic spine density brought on by E2.

Nucleus reuniens of the thalamus controls fear memory reconsolidation

The nucleus reuniens has been shown to support the acquisition, consolidation, maintenance, destabilization upon retrieval, and extinction of aversive memories. However, the direct participation of this thalamic subregion in memory reconsolidation is yet to be examined. The present study addressed this question in contextually fear-conditioned rats. Post-reactivation infusion of the GABA_A receptor agonist muscimol, the glutamate N2A-containing NMDA receptor antagonist TCN-201, or the protein synthesis inhibitor anisomycin into the NR induced significant impairments in memory reconsolidation. Administering muscimol or TCN-201 and anisomycin locally, or associating locally infused muscimol or TCN-201 with systemically administered clonidine, an α₂-receptor adrenergic agonist that attenuates the noradrenergic tonus associated with memory reconsolidation, produced no further reduction in freezing times when compared with the muscimol-vehicle, TCN-201-vehicle, vehicle-anisomycin, and vehicle-clonidine groups. This pattern of results indicates that such treatment combinations produced no additive/synergistic effects on reconsolidation. It is plausible that NR inactivation and antagonism of glutamate N2A-containing NMDA receptors weakened/prevented the subsequent action of anisomycin and clonidine because they disrupted the early stages of signal transduction pathways involved in memory reconsolidation. It is noteworthy that these pharmacological interventions, either alone or combined, induced no contextual memory specificity changes, as assessed in a later test in a novel and unpaired context. Besides, omitting memory reactivation precluded the impairing effects of muscimol, TCN-201, anisomycin, and clonidine on reconsolidation. Together, the present findings demonstrate interacting mechanisms through which the NR can regulate contextual fear memory restabilization.

Electroconvulsive Shock Does Not Impair the Reconsolidation of Cued and Contextual Pavlovian Threat Memory

Existing memories, when retrieved under certain circumstances, can undergo modification through the protein synthesis-dependent process of reconsolidation. Disruption of this process can lead to the weakening of a memory trace, an approach which is being examined as a potential treatment for disorders characterized by pathological memories, such as Post-Traumatic Stress Disorder. The success of this approach relies upon the ability to robustly attenuate reconsolidation; however, the available literature brings into question the reliability of the various drugs used to achieve such a blockade. The identification of a drug or intervention that can reliably disrupt reconsolidation without requiring intracranial access for administration would be extremely useful. Electroconvulsive shock (ECS) delivered after memory retrieval has been demonstrated in some studies to disrupt memory reconsolidation; however, there exists a paucity of literature characterizing its effects on Pavlovian fear memory. Considering this, we chose to examine ECS as an inexpensive and facile means to impair reconsolidation in rats. Here we show that electroconvulsive seizure induction, when administered after memory retrieval, (immediately, after 30 min, or after 1 h), does not impair the reconsolidation of cued or contextual Pavlovian fear memories. On the contrary, ECS administration immediately after extinction training may modestly impair the consolidation of fear extinction memory.

Cell-type-specific drug-inducible protein synthesis inhibition demonstrates that memory consolidation requires rapid neuronal translation

New protein synthesis is known to be required for the consolidation of memories, yet existing methods to block translation lack spatiotemporal precision and cell-type specificity, preventing investigation of cell-specific contributions of protein synthesis. Here, we developed a combined knock-in mouse and chemogenetic approach for cell type-specific and drug-inducible protein synthesis inhibition (ciPSI) that enables rapid and reversible phosphorylation of eIF2α, leading to inhibition of general translation by 50% in vivo. We use ciPSI to show that targeted protein synthesis inhibition pan-neuronally and in excitatory neurons in lateral amygdala (LA) impaired long-term memory. This could be recovered with artificial chemogenetic activation of LA neurons, though at the cost of stimulus generalization. Conversely, genetically reducing phosphorylation of eIF2α in excitatory neurons in LA enhanced memory strength, but reduced memory fidelity and behavioral flexibility. Our findings provide evidence for a cell-specific translation program during consolidation of threat memories.

Updating Procedures Can Reorganize the Neural Circuit Supporting a Fear Memory

Established memories undergo a period of vulnerability following retrieval, a process termed 'reconsolidation.' Recent work has shown that the hypothetical process of reconsolidation is only triggered when new information is presented during retrieval, suggesting that this process may allow existing memories to be modified. Reconsolidation has received increasing attention as a possible therapeutic target for treating disorders that stem from traumatic memories, yet little is known about how this process changes the original memory. In particular, it is unknown whether reconsolidation can reorganize the neural circuit supporting an existing memory after that memory is modified with new information. Here, we show that trace fear memory undergoes a protein synthesis-dependent reconsolidation process following exposure to a single updating trial of delay conditioning. Further, this reconsolidation-dependent updating process appears to reorganize the neural circuit supporting the trace-trained memory, so that it better reflects the circuit supporting delay fear. Specifically, after a trace-to-delay update session, the amygdala is now required for extinction of the updated memory but the retrosplenial cortex is no longer required for retrieval. These results suggest that updating procedures could be used to force a complex, poorly defined memory circuit to rely on a better-defined neural circuit that may be more amenable to behavioral or pharmacological manipulation. This is the first evidence that exposure to new information can fundamentally reorganize the neural circuit supporting an existing memory.

Emotional Modulation of Learning and Memory: Pharmacological Implications

Memory consolidation involves the process by which newly acquired information becomes stored in a long-lasting fashion. Evidence acquired over the past several decades, especially from studies using post-training drug administration, indicates that emotional arousal during the consolidation period influences and enhances the strength of the memory and that multiple different chemical signaling systems participate in this process. The mechanisms underlying the emotional influences on memory involve the release of stress hormones and activation of the basolateral amygdala, which work together to modulate memory consolidation. Moreover, work suggests that this amygdala-based memory modulation occurs with numerous types of learning and involves interactions with many different brain regions to alter consolidation. Additionally, studies suggest that emotional arousal and amygdala activity in particular influence synaptic plasticity and associated proteins in downstream brain regions. This review considers the historical understanding for memory modulation and cellular consolidation processes and examines several research areas currently using this foundational knowledge to develop therapeutic treatments.

Consolidation of altered associability information by amygdala central nucleus

The surprising omission of a reinforcer can enhance the associability of the stimuli that were present when the reward prediction error was induced, so that they more readily enter into new associations in the future. Previous research from this laboratory identified brain circuit elements critical to the enhancement of stimulus associability by the omission of an expected event and to the subsequent expression of that altered associability in more rapid learning. These elements include the amygdala, the midbrain substantia nigra, the basal forebrain substantia innominata, the dorsolateral striatum, the secondary visual cortex, and the posterior parietal cortex. Here, we found that consolidation of a surprise-enhanced associability memory in a serial prediction task depends on processing in the amygdala central nucleus (CeA) after completion of sessions that included the surprising omission of an expected event. Post-surprise infusions of anisomycin, lidocaine, or muscimol prevented subsequent display of surprise-enhanced associability. Because previous studies indicated that CeA function is unnecessary for the expression of associability enhancements that were induced previously when CeA function was intact (Holland & Gallagher, 2006), we interpreted these results as indicating that post-surprise activity of CeA ("surprise replay") is necessary for the consolidation of altered associability memories elsewhere in the brain, such as the posterior parietal cortex (Schiffino et al., 2014a).

Spermidine-induced improvement of reconsolidation of memory involves calcium-dependent protein kinase in rats

In this study, we determined whether the calcium-dependent protein kinase (PKC) signaling pathway is involved in the improvement of fear memory reconsolidation induced by the intrahippocampal administration of spermidine in rats. Male Wistar rats were trained in a fear conditioning apparatus using a 0.4-mA footshock as an unconditioned stimulus. Twenty-four hours after training, animals were re-exposed to the apparatus in the absence of shock (reactivation session). Immediately after the reactivation session, spermidine (2–200 pmol/site), the PKC inhibitor 3-[1-(dimethylaminopropyl)indol-3-yl]-4-(indol-3-yl) maleimide hydrochloride (GF 109203X, 0.3–30 pg/site), the antagonist of the polyamine-binding site at the NMDA receptor, arcaine (0.2–200 pmol/site), or the PKC activator phorbol 12-myristate 13-acetate (PMA, 0.02–2 nmol/site) was injected. While the post-reactivation administration of spermidine (20 and 200 pmol/site) and PMA (2 nmol/site) improved memory reconsolidation, GF 109203X (1, 10, and 30 pg/site) and arcaine (200 pmol/site) impaired it. GF 109203X (0.3 pg/site) impaired memory reconsolidation in the presence of spermidine (200 pmol/site). PMA (0.2 nmol/site) prevented the arcaine (200 pmol/site)-induced impairment of memory reconsolidation. Anisomycin (2 µg/site) also impaired memory reconsolidation in the presence of spermidine (200 pmol/site). Drugs had no effect when they were administered in the absence of reactivation. These results suggest that the spermidine-induced enhancement of memory reconsolidation involves PKC activation.

The retrosplenial cortex is involved in the formation of memory for context and trace fear conditioning

The retrosplenial cortex (RSC) is known to play a role in the retrieval of context memory, but its involvement in memory formation and consolidation is unclear. To better characterize the role of the RSC, we tested its involvement in the formation and retrieval of memory for trace fear conditioning, a task that requires the association of two cues separated by an empty period of time. We have previously shown that trace fear extinction requires the RSC (Kwapis, Jarome, Lee, Gilmartin, & Helmstetter, 2014) and have hypothesized that trace memory may be stored in a distributed cortical network that includes prelimbic and retrosplenial cortices (Kwapis, Jarome, & Helmstetter, 2015). Whether the RSC participates in acquiring and storing cued trace fear, however, is currently unknown. Here, we demonstrate that blocking protein synthesis in the RSC before, but not after acquisition impairs rats' memory for trace CS and context fear without affecting memory for the CS in standard delay fear conditioning. We also show that NMDA receptor blockade in the RSC transiently impairs memory retrieval for trace, but not delay memory. The RSC therefore appears to critically contribute to formation of trace and context fear memory in addition to its previously recognized role in context memory retrieval.

Regulation of peroxisome proliferator-activated receptors (PPAR) α and -γ of rat brain astrocytes in the course of activation by toll-like receptor agonists

Peroxisome proliferator-activated receptors (PPAR)-α and -γ in astrocytes play important roles in inflammatory brain pathologies. Understanding the regulation of both activity and expression levels of PPARs is an important neuroscience issue. Toll-like receptor (TLR) agonists are inflammatory stimuli that could modulate PPAR, but the mechanisms of their control in astrocytes are poorly understood. In the present study, we report that lipopolysaccharide, peptidoglycan, and flagellin, which are agonists of TLR4, TLR1/2, and TLR5, respectively, exert time- and nuclear factor kappa-light-chain-enhancer of activated B cells-dependent suppression of mRNA, protein and activity of PPARα and PPARγ. In naïve astrocytes, PPARα and PPARγ mRNA have short turnover time (half-life about 30 min for PPARα, 75 min for PPARγ) with a nearly two-fold stabilization after TLR-activation. p38 inhibition abolished TLR-induced stabilization. The levels of PPARα and PPARγ mRNA, and protein and DNA-binding activity could be modified using c-Jun N-terminal Kinase and p38 inhibitors. In addition, the expression levels of both PPARα and PPARγ isotypes were induced after inhibition of protein synthesis. This induction signifies participation of additional regulatory proteins with short life-time. They are p38-sensitive for PPARα and c-Jun N-terminal Kinase-sensitive for PPARγ. Thus, PPARα and PPARγ are regulated in astrocytes on mRNA and protein levels, mRNA stability, and DNA-binding activity during TLR-mediated responses. Astrocytes have the triad of PPARα, PPARβ/δ, and PPARγ in regulation of proinflammatory responses. Activation of Toll-like receptors (TLR) leads to PPARβ/δ overexpression, PPARα and PPARγ suppression via TLR/NF-κB pathway on mRNA, protein and activity levels. Mitogen-activated protein kinases (MAPK) p38 and JNK are involved in regulation of PPAR expression. p38 MAPK plays a special role in stabilization of PPAR mRNA.

The amygdala is critical for trace, delay, and contextual fear conditioning

Numerous investigations have definitively shown amygdalar involvement in delay and contextual fear conditioning. However, much less is known about amygdala contributions to trace fear conditioning, and what little evidence exists is conflicting as noted in previous studies. This discrepancy may result from selective targeting of individual nuclei within the amygdala. The present experiments further examine the contributions of amygdalar subnuclei to trace, delay, and contextual fear conditioning. Rats were trained using a 10-trial trace, delay, or unpaired fear conditioning procedure. Pretraining lesions targeting the entire basolateral amygdala (BLA) resulted in a deficit in trace, delay, and contextual fear conditioning. Immediate post-training infusions of the protein synthesis inhibitor, cycloheximide, targeting the basal nucleus of the amygdala (BA) attenuated trace and contextual fear memory expression, but had no effect on delay fear conditioning. However, infusions targeting the lateral nucleus of the amygdala (LA) immediately following conditioning attenuated contextual fear memory expression, but had no effect on delay or trace fear conditioning. In follow-up experiments, rats were trained using a three-trial delay conditioning procedure. Immediate post-training infusions targeting the LA produced deficits in both delay tone and context fear, while infusions targeting the BA produced deficits in context but not delay tone fear. These data fully support a role for the BLA in trace, delay, and contextual fear memories. Specifically, these data suggest that the BA may be more critical for trace fear conditioning, whereas the LA may be more critical for delay fear memories.

Molecular mechanisms underlying formation of long-term reward memories and extinction memories in the honeybee (Apis mellifera)

The honeybee (Apis mellifera) has long served as an invertebrate model organism for reward learning and memory research. Its capacity for learning and memory formation is rooted in the ecological need to efficiently collect nectar and pollen during summer to ensure survival of the hive during winter. Foraging bees learn to associate a flower's characteristic features with a reward in a way that resembles olfactory appetitive classical conditioning, a learning paradigm that is used to study mechanisms underlying learning and memory formation in the honeybee. Due to a plethora of studies on appetitive classical conditioning and phenomena related to it, the honeybee is one of the best characterized invertebrate model organisms from a learning psychological point of view. Moreover, classical conditioning and associated behavioral phenomena are surprisingly similar in honeybees and vertebrates, suggesting a convergence of underlying neuronal processes, including the molecular mechanisms that contribute to them. Here I review current thinking on the molecular mechanisms underlying long-term memory (LTM) formation in honeybees following classical conditioning and extinction, demonstrating that an in-depth analysis of the molecular mechanisms of classical conditioning in honeybees might add to our understanding of associative learning in honeybees and vertebrates.

Post-retrieval late process contributes to persistence of reactivated fear memory

Several studies have demonstrated the mechanisms involved in memory persistence after learning. However, little is known about memory persistence after retrieval. In this study, a protein synthesis inhibitor, anisomycin, was infused into the basolateral amygdala of mice 9.5 h after retrieval of contextual conditioned fear. Anisomycin attenuated fear memory after 7 d, but not after 2 d. In contrast, infusion of anisomycin 5- or 24-h post-retrieval was ineffective. These findings indicate that anisomycin attenuates the persistence of reactivated fear memory in a time-dependent manner. We propose that late protein synthesis is required for memory persistence after retrieval.

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.

Memory consolidation in both trace and delay fear conditioning is disrupted by intra-amygdala infusion of the protein synthesis inhibitor anisomycin

Memory for delay fear conditioning requires the synthesis of new mRNA and protein in the basolateral amygdala. It is currently unknown whether similar molecular processes in the amygdala are required for the formation of trace fear memory, in which a stimulus-free interval is inserted between the conditional stimulus (CS) and unconditional stimulus (UCS). Here, we show that infusion of the protein synthesis inhibitor anisomycin into the basolateral amygdala disrupts consolidation of both trace and delay fear conditioning. This is the first evidence that protein synthesis in the amygdala is necessary for the formation of both trace and delay fear memory.

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.

p38 mitogen-activated protein kinase and PI3-kinase are involved in up-regulation of mu opioid receptor transcription induced by cycloheximide

Despite several decades of efforts to develop safer, efficacious, and non-addictive opioids for pain treatment, morphine remains the most valuable painkiller in contemporary medicine. Morphine and endogenous mu opioid peptides exert their pharmacological actions mainly through the mu opioid receptor (MOR). Analgesic effects of opioids in animals are dependent on the MOR expression levels, as demonstrated by studies of MOR-knockout mice (homo/heterozygotes) and MOR-less expressing mice. Surprisingly, in the course of our investigation to understand the mechanisms involved in the regulation of MOR gene expression, cycloheximide (CHX), a known protein synthesis inhibitor, markedly induced accumulation of MOR mRNAs in both MOR-negative and -positive cells. This induction was blocked by inhibitors of phosphoinositide 3-kinase (PI3-K) and p38 MAPK, but not by a p42/44 MAPK inhibitor. In vitro, CHX was found to activate the MOR promoter and this activation was suppressed by inhibition of PI3-K. The transcriptional activator Sox18 was recruited to the MOR promoter in CHX-treated cells and this recruitment was also inhibited by the PI3-K and p38 MAPK inhibitors, Ly294002 and SB203580, respectively. Consistently, acetylation of histone H3 and induction of H3-K4 methylation were detected while reductions of histone deacetylase 2 binding and H3-K9 methylation were observed on the promoter. Furthermore, the MOR mRNA accumulation was almost completely inhibited in the presence of actinomycin-D, indicating that this effect occurs mainly through activation of the transcriptional machinery. These observations suggest that CHX directly induces MOR gene transcription by recruiting the active transcription factor Sox18 to the MOR promoter through PI3- and/or p38 MAPK pathways.

Cycloheximide-mediated superinduction of genes involves both native and foreign transcripts in rice (Oryza sativa L.)

Rice seedlings subjected to heat shock show rapid and transient induction of Oshsp17.4-CI, Oshsp17.9A-CI and OsClpB-cyt/hsp100 transcripts. When the seedlings were pre-treated with protein synthesis inhibitor cycloheximide, levels of the above transcripts during heat shock were more elevated than those seen with heat shock alone. Heat stress and cycloheximide co-treatment resulted in higher transcript accumulation in comparison to cycloheximide pre-treatment followed by heat stress. In transgenic plants raised with OsClpB-cyt/hsp100 promoter driving expression of the reporter gus gene, expression of the gus transcript was subjected to similar superinduction event as was seen with native OsClpB-cyt/hsp100 transcripts in untransformed plants. Yeast cells transformed with variably-sized rice ClpB-cyt/hsp100 promoter driving expression of the lacZ reporter transcript showed that specific sequences of the promoter region may be implicated in superinduction.

Molecular specificity of multiple hippocampal processes governing fear extinction

Over many years, fear extinction has been conceptualized as one dominant process, new inhibitory learning, which serves to dampen previously acquired fear. Here we present an alternative view, that brain region-specific processing of representations, expectations and emotional attributes of the fear-provoking event, recruits unique mechanisms that interdependently contribute to the conditioning and extinction of fear. The co-occurrence of these mechanisms within the fear circuit can thus be tracked and differentiated at a molecular and cellular level. Among others, the transcriptional regulators cFos, cAMP-dependent response element binding protein (CREB), Zif268, and extracellular signal-regulated kinases (Erk) stand out as hippocampal nuclear markers signaling novelty, arousal, retrieval, and prediction error, respectively. Consistent with evidence from human studies, these findings indicate that, beyond inhibitory learning, fear extinction requires modification of the emotional attributes and expectations that define the threatening context. Given the likely dysregulation of one or more of these processes in anxiety disorders, a key research challenge for the future is the identification and enhancement of individual extinction mechanisms to target the specific components of fear. Environmental stimuli lacking affective properties (conditioned stimuli, CS) rapidly become threatening if presented with stressful events (unconditioned stimuli, US). Consequently, based on a CS-US association, the presentation of the CS triggers species-specific fear responses until the US consistently stops occurring. At that point, new learning takes place and the fear response declines, a phenomenon termed extinction. The view that extinction occurs because a new, inhibitory CS-noUS association gains control over behavior, has remained dominant in the field. The implications of impaired fear regulation in the development of anxiety disorders have stimulated intense research in this area. Rodent studies identified the circuits involved in the conditioning and extinction of fear of salient cues, generating data that were confirmed in humans with brain imaging approaches. Nevertheless, research with experimental animals has not fully taken advantage of human data in order to better interpret extinction mechanisms in the framework of learning, expectation and emotion governing fear-motivated behavior. The present article aims to summarize recent molecular evidence on fear extinction, focusing on hippocampal mechanisms and experimental models of contextual fear, and compare the results with other relevant fear paradigms and human imaging studies. Instead of conceptualizing extinction learning as one process, such as CS-noUS association or inhibitory learning, we propose that fear extinction reflects the behavioral output of several region-specific learning processes that modify different components of the conditioning memory. The significance of these findings is discussed in the framework of fear regulation and anxiety disorders.

Pain and learning in a spinal system: contradictory outcomes from common origins

The long-standing belief that the spinal cord serves merely as a conduit for information traveling to and from the brain is changing. Over the past decade, research has shown that the spinal cord is sensitive to response-outcome contingencies, demonstrating that spinal circuits have the capacity to modify behavior in response to differential environmental cues. If spinally transected rats are administered shock contingent on leg extension (controllable shock), they will maintain a flexion response that minimizes shock exposure. If, however, this contingency is broken, and shock is administered irrespective of limb position (uncontrollable shock), subjects cannot acquire the same flexion response. Interestingly, each of these treatments has a lasting effect on behavior; controllable shock enables future learning, while uncontrollable shock produces a long-lasting learning deficit. Here we suggest that the mechanisms underlying learning and the deficit may have evolved from machinery responsible for the spinal processing of noxious information. Experiments have shown that learning and the deficit require receptors and signaling cascades shown to be involved in central sensitization, including activation of NMDA and neurokinin receptors, as well as CaMKII. Further supporting this link between pain and learning, research has also shown that uncontrollable stimulation results in allodynia. Moreover, systemic inflammation and neonatal hindpaw injury each facilitate pain responding and undermine the ability of the spinal cord to support learning. These results suggest that the plasticity associated with learning and pain must be placed in a balance in order for adaptive outcomes to be observed.

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.

Approach memory turns to avoidance memory with age

Ontogenetic modification of an early memory is relatively poorly understood. And an important question is whether the memory output is more determined by the age at acquisition or at retention? Here we explore the expression of odor-shock conditioning in the rat pup. Acquisition at post-natal day 6 (P6) leads to an approach response and at post-natal day 12 (P12) to an avoidance response when the retention test is 24h later. In both cases, anisomycin injected immediately post-acquisition induced a retrograde amnesia. Controls show that, in either case, short-term memory measured 4h after acquisition is not impaired and that anisomycin given after a 4h delay has no effect. Thus, at the two ages, memory involves a consolidation process. The main result is the spontaneous reversal of the conditioned response from approach acquired at P6 to avoidance when tested at P13. This phenomenon is robust as it is observed in three conditions. Moreover, amnesia induced at P6 is maintained at P13. Results are discussed in terms of maturation and/or competition of the memory traces.

What does it take to demonstrate memory erasure? Theoretical comment on Norrholm et al. (2008)

An issue of increasing interest in Pavlovian conditioning is to identify ways to facilitate the development and persistence of extinction. Both behavioral and molecular lines of evidence demonstrate that learning during extinction can be enhanced. Similar evidence has been offered to support the idea that extinction causes the original association to be unlearned, or erased. Differentiating between extinction and erasure accounts is extremely difficult and requires many assumptions about the fundamental nature of how memory storage maps into memory expression. In this issue of Behavioral Neuroscience, Norrholm et al. (see record 2008-13280-002) describe a study of extinction with humans that has the potential to serve as a translational bridge between rodent work and clinical applications. They find less recovery of a conditioned fear response when extinction occurs 10 min compared with 72 hr after conditioning; however, the recovery of subjects' expectancies of the fearful stimulus is independent of when extinction occurred. These findings and others discussed in this article demonstrate some of the challenges in making inferences about memory erasure during extinction.

The temporal dynamics of consolidation and reconsolidation decrease during postnatal development

The temporal dynamics of consolidation and reconsolidation of taste/odor aversion memory are evaluated during rat pup growth at postnatal days 3, 10, and 18. This is assessed through the temporal gradients of efficacy of a protein synthesis inhibitor (anisomycin) in inducing amnesia after either acquisition (consolidation) or reactivation (reconsolidation). The results show a progressive reduction with age of the delay during which the inhibitor is able to induce amnesia. Control experiments rule out a reduction of anisomycin efficacy due to blood brain barrier growth or decrease in protein synthesis inhibition. Thus, these results present the first evidence that the protein synthesis-dependent phase of memory stabilization requires less time with age. This decrease occurs in parallel for consolidation and reconsolidation. Such changes in the dynamics of memory processing could contribute to the cognitive improvement associated with development.

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.