PKA has a critical role in synaptic delivery of GluR1- and GluR4-containing AMPARs during initial stages of acquisition of in vitro classical conditioning

Synaptic Mechanisms of Delay Eyeblink Classical Conditioning: AMPAR Trafficking and Gene Regulation in an In Vitro Model

An in vitro model of delay eyeblink classical conditioning was developed to investigate synaptic plasticity mechanisms underlying acquisition of associative learning. This was achieved by replacing real stimuli, such as an airpuff and tone, with patterned stimulation of the cranial nerves using an isolated brainstem preparation from turtle. Here, our primary findings regarding cellular and molecular mechanisms for learning acquisition using this unique approach are reviewed. The neural correlate of the in vitro eyeblink response is a replica of the actual behavior, and features of conditioned responses (CRs) resemble those observed in behavioral studies. Importantly, it was shown that acquisition of CRs did not require the intact cerebellum, but the appropriate timing did. Studies of synaptic mechanisms indicate that conditioning involves two stages of AMPA receptor (AMPAR) trafficking. Initially, GluA1-containing AMPARs are targeted to synapses followed later by replacement by GluA4 subunits that support CR expression. This two-stage process is regulated by specific signal transduction cascades involving PKA and PKC and is guided by distinct protein chaperones. The expression of the brain-derived neurotrophic factor (BDNF) protein is central to AMPAR trafficking and conditioning. BDNF gene expression is regulated by coordinated epigenetic mechanisms involving DNA methylation/demethylation and chromatin modifications that control access of promoters to transcription factors. Finally, a hypothesis is proposed that learning genes like BDNF are poised by dual chromatin features that allow rapid activation or repression in response to environmental stimuli. These in vitro studies have advanced our understanding of the cellular and molecular mechanisms that underlie associative learning.

Emergence of In Vitro Preparations and Their Contribution to Understanding the Neural Control of Behavior in Vertebrates

One of the longstanding goals of the field of neuroscience is to understand the neural control of behavior in both invertebrate and vertebrate species. A series of early discoveries showed that certain motor patterns like locomotion could be generated by neuronal circuits without sensory feedback or descending control systems. These were called fictitious, or "fictive", motor programs because they could be expressed by neurons in the absence of movement. This finding lead investigators to isolate central nervous system tissue and maintain it in a dish in vitro to better study mechanisms of motor pattern generation. A period of rapid development of in vitro preparations from invertebrate species that could generate fictive motor programs from the activity of central pattern generating circuits (CPGs) emerged that was gradually followed by the introduction of such preparations from vertebrates. Here, I will review some of the notable in vitropreparations from both mammalian and non-mammalian vertebrate species developed to study the neural circuits underlying a variety of complex behaviors. This approach has been instrumental in delineating not only the cellular substrates underlying locomotion, respiration, scratching, and other behaviors, but also mechanisms underlying the modifiability of motor pathways through synaptic plasticity. In vitro preparations have had a significant impact on the field of motor systems neuroscience and the expansion of our understanding of how nervous systems control behavior. The field is ready for further advancement of this approach to explore neural substrates for variations in behavior generated by social and seasonal context, and the environment.

Effects Of Biotin Deficiency On Short Term Memory: The Role Of Glutamate, Glutamic Acid, Dopamine And Protein Kinase A

Insufficient dietary biotin intake, biotinidase deficiency, drug-biotin interactions can cause biotin deficiency which may result in central nervous system dysfunctions. We hypothesized that biotin deficiency could disrupt learning and memory functions by altering glutamate, glutamine, dopamine levels and protein kinase A (PKA) activity in the hippocampus. Sixteen female and 4 male Wistar rats were mated and females were separated into 4 groups. Three pups were selected from each mother and a total of 48 pups were divided into the following experimental groups. NN group, normal diet in the prenatal and postnatal period. NB group, normal diet in the prenatal and a biotin-deficient diet in the postnatal period. BN group: biotin-deficient diet in the prenatal and a normal diet in the postnatal period, BB group: biotin-deficient diet in both the prenatal and postnatal period. Open Field, Y-Maze, Object Location, and Novel Object Recognition Tests were performed in all groups and rats were sacrificed. Glutamine, glutamate, dopamine levels and PKA activity were analyzed in the hippocampus. In the open field test, distance and velocity values of NB, BN and BB groups were decreased with respect to the NN group. Learning and memory functions of NB, BN and BB groups were found to be impaired in behavioral tests. Dopamine levels and PKA activity were also decreased in all rat pups fed with a biotin deficient diet. In conclusion, we demonstrated that biotin deficiency deteriorates short-term memory and locomotor activity. This impairment may relate to decreased dopamine levels and PKA activity in the hippocampus.

Regulation of AMPAR trafficking in synaptic plasticity by BDNF and the impact of neurodegenerative disease

Research demonstrates that the neural mechanisms underlying synaptic plasticity and learning and memory involve mobilization of AMPA-type neurotransmitter receptors at glutamatergic synaptic contacts, and that these mechanisms are targeted during neurodegenerative disease. Strengthening neural transmission occurs with insertion of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) into synapses while weakening results from receptor withdrawal. A key player in the trafficking of AMPARs during plasticity and learning is the brain-derived neurotrophic factor (BDNF) signaling system. BDNF is a neurotrophic factor that supports neuronal growth and is required for learning and memory. Significantly, a primary feature of many neurodegenerative diseases is a reduction in BDNF protein as well as disrupted neuronal surface expression of synaptic AMPARs. The resulting weakening of synaptic contacts leads to synapse loss and neuronal degeneration that underlies the cognitive impairment and dementia observed in patients with progressive neurodegenerative disease such as Alzheimer's. In the face of these data, one therapeutic approach is to increase BDNF bioavailability in brain. While this has been met with significant challenges, the results of the research have been promising. In spite of this, there are currently no clinical trials to test many of these findings on patients. Here, research showing that BDNF drives AMPARs to synapses, AMPAR trafficking is essential for synaptic plasticity and learning, and that neurodegenerative disease results in a significant decline in BDNF will be reviewed. The aim is to draw attention to the need for increasing patient-directed clinical studies to test the possible benefits of increasing levels of neurotrophins, specifically BDNF, to treat brain disorders. Much is known about the cellular mechanisms that underlie learning and memory in brain. It can be concluded that signaling by neurotrophins like BDNF and AMPA-type glutamate receptor synaptic trafficking are fundamental to these processes. Data from animal models and patients reveal that these mechanisms are adversely targeted during neurodegenerative disease and results in memory loss and cognitive decline. A brief summary of our understanding of these mechanisms indicates that it is time to apply this knowledge base directly to development of therapeutic treatments that enhance neurotrophins for brain disorders in patient populations.

Mapping Phosphodiesterase 4D (PDE4D) in Macaque Dorsolateral Prefrontal Cortex: Postsynaptic Compartmentalization in Layer III Pyramidal Cell Circuits

cAMP signaling has powerful, negative effects on cognitive functions of the primate dorsolateral prefrontal cortex (dlPFC), opening potassium channels to reduce firing and impair working memory, and increasing tau phosphorylation in aging neurons. This contrasts with cAMP actions in classic circuits, where it enhances plasticity and transmitter release. PDE4 isozymes regulate cAMP actions, and thus have been a focus of research and drug discovery. Previous work has focused on the localization of PDE4A and PDE4B in dlPFC, but PDE4D is also of great interest, as it is the predominant PDE4 isoform in primate association cortex, and PDE4D expression decreases with aging in human dlPFC. Here we used laser-capture microdissection transcriptomics and found that PDE4D message is enriched in pyramidal cells compared to GABAergic PV-interneurons in layer III of the human dlPFC. A parallel study in rhesus macaques using high-spatial resolution immunoelectron microscopy revealed the ultrastructural locations of PDE4D in primate dlPFC with clarity not possible in human post-mortem tissue. PDE4D was especially prominent in dendrites associated with microtubules, mitochondria, and likely smooth endoplasmic reticulum (SER). There was substantial postsynaptic labeling in dendritic spines, associated with the SER spine-apparatus near glutamatergic-like axospinous synapses, but sparse labeling in axon terminals. We also observed dense PDE4D labeling perisynaptically in astroglial leaflets ensheathing glutamatergic connections. These data suggest that PDE4D is strategically positioned to regulate cAMP signaling in dlPFC glutamatergic synapses and circuits, especially in postsynaptic compartments where it is localized to influence cAMP actions on intracellular trafficking, mitochondrial physiology, and internal calcium release.

Learning experiences comprising central ethanol exposure in rat neonates: Impact upon respiratory plasticity and the activity of brain catalase

Fetal ethanol exposure represents a risk factor for sudden infant death syndrome, and the respiratory effects of fetal ethanol exposure promote hypoxic ischemic consequences. This study analyzes central ethanol's effects upon breathing plasticity during an ontogenetic stage equivalent to the human third gestational trimester. Ethanol's unconditioned breathing effects and their intervention in learning processes were examined. Since central ethanol is primarily metabolized via the catalase system, we also examined the effects of early history with the drug upon this system. During postnatal days 3, 5, and 7 (PDs 3-7), pups were intracisternally administered with vehicle or ethanol (300 mg%). They were tested in a plethysmograph scented or not scented with ethanol odor. The state of intoxication attenuated the onset of apneas, a phenomenon that is suggestive of ethanol's anxiolytic effects given the state of arousal caused by the novel environment and the stress of ethanol administration. At PD9, pups were evaluated when sober under sequential air conditions (initial-normoxia, hypoxia, and recovery-normoxia), with or without the presence of ethanol odor. Initial apneic episodes increased when ethanol intoxication was previously associated with the odor. Pups then ingested ethanol, and brain catalase activity was determined. Pre-exposure to ethanol intoxication paired with the odor of the drug resulted in heightened enzymatic activity. Central ethanol exposure appears to exert antianxiety effects that attenuate apneic disruptions. However, during withdrawal, the cues associated with such effects elicit an opposite reaction. The activity of the catalase system was also dependent upon learning processes that involved the association of environmental stimuli and ethanol intoxication.

Ubiquitin biology in neurodegenerative disorders: From impairment to therapeutic strategies

The abnormal accumulation of neurotoxic proteins is the typical hallmark of various age-related neurodegenerative disorders (NDDs), including Alzheimer's disease, Parkinson's disease, Huntington's disease, Amyotrophic lateral sclerosis and Multiple sclerosis. The anomalous proteins, such as Aβ, Tau in Alzheimer's disease and α-synuclein in Parkinson's disease, perturb the neuronal physiology and cellular homeostasis in the brain thereby affecting the millions of human lives across the globe. Here, ubiquitin proteasome system (UPS) plays a decisive role in clearing the toxic metabolites in cells, where any aberrancy is widely reported to exaggerate the neurodegenerative pathologies. In spite of well-advancement in the ubiquitination research, their molecular markers and mechanisms for target-specific protein ubiquitination and clearance remained elusive. Therefore, this review substantiates the role of UPS in the brain signaling and neuronal physiology with their mechanistic role in the NDD's specific pathogenic protein clearance. Moreover, current and future promising therapies are discussed to target UPS-mediated neurodegeneration for better public health.

Microglia CREB-Phosphorylation Mediates Amyloid-β-Induced Neuronal Toxicity

Extracellular accumulation of amyloid-β (Aβ) forming senile plaques is one of the hallmark pathologies in Alzheimer's disease (AD), while the mechanisms underlying the neuronal toxic effect of Aβ are not fully understood. Here, we found that intracerebroventricular infusion of the aged Aβ42 in mice only induces memory deficit at 24 h but not at 7 days. Interestingly, a remarkably increased CREB (cAMP response element-binding protein) Ser133-phosphorylation (pS133-CREB) with microglial activation was detected at 24 h but not at 7 days after Aβ infusion. Aβ treatment for 24 h increased pS133-CREB level in microglia of the hippocampal non-granular cell layers with remarkably decreased pS133-CREB immunoreactivity in neurons of the hippocampal granular cell layers, including CA1, CA3, and DG subsets. Inhibition of microglia activation by minocycline or CREB phosphorylation by H89, an inhibitor of protein kinase A (PKA), abolished Aβ-induced microglia CREB hyperphosphorylation with restoration of neuronal function and attenuation of inflammatory response, i.e., reduced levels of interleukin-6 (IL6) and pCREB binding of matrix metalloproteinase-9 (MMP9) DNA. Finally, treatment of the primary hippocampal neurons with Aβ-potentiated microglia media decreased neuronal GluN1 and GluA2 levels, while simultaneous inhibition of PKA restored the levels. These novel findings reveal that intracerebroventricular infusion of Aβ only induces transient memory deficit in mice and the molecular mechanisms involve a stimulated microglial CREB phosphorylation.

Input from the medial geniculate nucleus modulates amygdala encoding of fear memory discrimination

Generalization of fear can involve abnormal responding to cues that signal safety and is common in people diagnosed with post-traumatic stress disorder. Differential auditory fear conditioning can be used as a tool to measure changes in fear discrimination and generalization. Most prior work in this area has focused on elevated amygdala activity as a critical component underlying generalization. The amygdala receives input from auditory cortex as well as the medial geniculate nucleus (MgN) of the thalamus, and these synapses undergo plastic changes in response to fear conditioning and are major contributors to the formation of memory related to both safe and threatening cues. The requirement for MgN protein synthesis during auditory discrimination and generalization, as well as the role of MgN plasticity in amygdala encoding of discrimination or generalization, have not been directly tested. GluR1 and GluR2 containing AMPA receptors are found at synapses throughout the amygdala and their expression is persistently up-regulated after learning. Some of these receptors are postsynaptic to terminals from MgN neurons. We found that protein synthesis-dependent plasticity in MgN is necessary for elevated freezing to both aversive and safe auditory cues, and that this is accompanied by changes in the expressions of AMPA receptor and synaptic scaffolding proteins (e.g., SHANK) at amygdala synapses. This work contributes to understanding the neural mechanisms underlying increased fear to safety signals after stress.

Regulation of BDNF chromatin status and promoter accessibility in a neural correlate of associative learning

Brain-derived neurotrophic factor (BDNF) gene expression critically controls learning and its aberrant regulation is implicated in Alzheimer's disease and a host of neurodevelopmental disorders. The BDNF gene is target of known DNA regulatory mechanisms but details of its activity-dependent regulation are not fully characterized. We performed a comprehensive analysis of the epigenetic regulation of the turtle BDNF gene (tBDNF) during a neural correlate of associative learning using an in vitro model of eye blink classical conditioning. Shortly after conditioning onset, the results from ChIP-qPCR show conditioning-dependent increases in methyl-CpG-binding protein 2 (MeCP2) and repressor basic helix-loop-helix binding protein 2 (BHLHB2) binding to tBDNF promoter II that corresponds with transcriptional repression. In contrast, enhanced binding of ten-eleven translocation protein 1 (Tet1), extracellular signal-regulated kinase 1/2 (ERK1/2), and cAMP response element-binding protein (CREB) to promoter III corresponds with transcriptional activation. These actions are accompanied by rapid modifications in histone methylation and phosphorylation status of RNA polymerase II (RNAP II). Significantly, these remarkably coordinated changes in epigenetic factors for two alternatively regulated tBDNF promoters during conditioning are controlled by Tet1 and ERK1/2. Our findings indicate that Tet1 and ERK1/2 are critical partners that, through complementary functions, control learning-dependent tBDNF promoter accessibility required for rapid transcription and acquisition of classical conditioning.

Acoustic trauma slows AMPA receptor-mediated EPSCs in the auditory brainstem, reducing GluA4 subunit expression as a mechanism to rescue binaural function

KEY POINTS

Lateral superior olive (LSO) principal neurons receive AMPA receptor (AMPAR) - and NMDA receptor (NMDAR)-mediated EPSCs and glycinergic IPSCs. Both EPSCs and IPSCs have slow kinetics in prehearing animals, which during developmental maturation accelerate to sub-millisecond decay time-constants. This correlates with a change in glutamate and glycine receptor subunit composition quantified via mRNA levels. The NMDAR-EPSCs accelerate over development to achieve decay time-constants of 2.5 ms. This is the fastest NMDAR-mediated EPSC reported. Acoustic trauma (AT, loud sounds) slow AMPAR-EPSC decay times, increasing GluA1 and decreasing GluA4 mRNA. Modelling of interaural intensity difference suggests that the increased EPSC duration after AT shifts interaural level difference to the right and compensates for hearing loss. Two months after AT the EPSC decay times recovered to control values. Synaptic transmission in the LSO matures by postnatal day 20, with EPSCs and IPSCs having fast kinetics. AT changes the AMPAR subunits expressed and slows the EPSC time-course at synapses in the central auditory system.

ABSTRACT

Damaging levels of sound (acoustic trauma, AT) diminish peripheral synapses, but what is the impact on the central auditory pathway? Developmental maturation of synaptic function and hearing were characterized in the mouse lateral superior olive (LSO) from postnatal day 7 (P7) to P96 using voltage-clamp and auditory brainstem responses. IPSCs and EPSCs show rapid acceleration during development, so that decay kinetics converge to similar sub-millisecond time-constants (τ, 0.87 ± 0.11 and 0.77 ± 0.08 ms, respectively) in adult mice. This correlated with LSO mRNA levels for glycinergic and glutamatergic ionotropic receptor subunits, confirming a switch from Glyα2 to Glyα1 for IPSCs and increased expression of GluA3 and GluA4 subunits for EPSCs. The NMDA receptor (NMDAR)-EPSC decay τ accelerated from >40 ms in prehearing animals to 2.6 ± 0.4 ms in adults, as GluN2C expression increased. In vivo induction of AT at around P20 disrupted IPSC and EPSC integration in the LSO, so that 1 week later the AMPA receptor (AMPAR)-EPSC decay was slowed and mRNA for GluA1 increased while GluA4 decreased. In contrast, GlyR IPSC and NMDAR-EPSC decay times were unchanged. Computational modelling confirmed that matched IPSC and EPSC kinetics are required to generate mature interaural level difference functions, and that longer-lasting EPSCs compensate to maintain binaural function with raised auditory thresholds after AT. We conclude that LSO excitatory and inhibitory synaptic drive matures to identical time-courses, that AT changes synaptic AMPARs by expression of subunits with slow kinetics (which recover over 2 months) and that loud sounds reversibly modify excitatory synapses in the brain, changing synaptic function for several weeks after exposure.

Coincidence detection in a neural correlate of classical conditioning is initiated by bidirectional 3-phosphoinositide-dependent kinase-1 signalling and modulated by adenosine receptors

How the neural substrates for detection of paired stimuli are distinct from unpaired stimuli is poorly understood and a fundamental question for understanding the signalling mechanisms for coincidence detection during associative learning. To address this question, we used a neural correlate of eyeblink classical conditioning in an isolated brainstem from the turtle, in which the cranial nerves are directly stimulated in place of using a tone or airpuff. A bidirectional response is activated in <5 min of training, in which phosphorylated 3-phosphoinositide-dependent kinase-1 (p-PDK1) is increased in response to paired and decreased in response to unpaired nerve stimulation and is mediated by the opposing actions of neurotrophin receptors TrkB and p75(NTR) . Surprisingly, blockade of adenosine 2A (A2A ) receptors inhibits both of these responses. Pairing also induces substantially increased surface expression of TrkB that is inhibited by Src family tyrosine kinase and A2A receptor antagonists. Finally, the acquisition of conditioning is blocked by a PDK1 inhibitor. The unique action of A2A receptors to function directly as G proteins and in receptor transactivation to control distinct TrkB and p75(NTR) signalling pathways allows for convergent activation of PDK1 and protein kinase A during paired stimulation to initiate classical conditioning.

Cyclophilin D deficiency rescues Aβ-impaired PKA/CREB signaling and alleviates synaptic degeneration

The coexistence of neuronal mitochondrial pathology and synaptic dysfunction is an early pathological feature of Alzheimer's disease (AD). Cyclophilin D (CypD), an integral part of mitochondrial permeability transition pore (mPTP), is involved in amyloid beta (Aβ)-instigated mitochondrial dysfunction. Blockade of CypD prevents Aβ-induced mitochondrial malfunction and the consequent cognitive impairments. Here, we showed the elimination of reactive oxygen species (ROS) by antioxidants probucol or superoxide dismutase (SOD)/catalase blocks Aβ-mediated inactivation of protein kinase A (PKA)/cAMP regulatory-element-binding (CREB) signal transduction pathway and loss of synapse, suggesting the detrimental effects of oxidative stress on neuronal PKA/CREB activity. Notably, neurons lacking CypD significantly attenuate Aβ-induced ROS. Consequently, CypD-deficient neurons are resistant to Aβ-disrupted PKA/CREB signaling by increased PKA activity, phosphorylation of PKA catalytic subunit (PKA C), and CREB. In parallel, lack of CypD protects neurons from Aβ-induced loss of synapses and synaptic dysfunction. Furthermore, compared to the mAPP mice, CypD-deficient mAPP mice reveal less inactivation of PKA-CREB activity and increased synaptic density, attenuate abnormalities in dendritic spine maturation, and improve spontaneous synaptic activity. These findings provide new insights into a mechanism in the crosstalk between the CypD-dependent mitochondrial oxidative stress and signaling cascade, leading to synaptic injury, functioning through the PKA/CREB signal transduction pathway.

Systems biology of synaptic plasticity: a review on N-methyl-D-aspartate receptor mediated biochemical pathways and related mathematical models

Synaptic plasticity, an emergent property of synaptic networks, has shown strong correlation to one of the essential functions of the brain, memory formation. Through understanding synaptic plasticity, we hope to discover the modulators and mechanisms that trigger memory formation. In this paper, we first review the well understood modulators and mechanisms underlying N-methyl-D-aspartate receptor dependent synaptic plasticity, a major form of synaptic plasticity in hippocampus, and then comment on the key mathematical modelling approaches available in the literature to understand synaptic plasticity as the integration of the established functionalities of synaptic components.

Sequential delivery of synaptic GluA1- and GluA4-containing AMPA receptors (AMPARs) by SAP97 anchored protein complexes in classical conditioning

Multiple signaling pathways are involved in AMPAR trafficking to synapses during synaptic plasticity and learning. The mechanisms for how these pathways are coordinated in parallel but maintain their functional specificity involves subcellular compartmentalization of kinase function by scaffolding proteins, but how this is accomplished is not well understood. Here, we focused on characterizing the molecular machinery that functions in the sequential synaptic delivery of GluA1- and GluA4-containing AMPARs using an in vitro model of eyeblink classical conditioning. We show that conditioning induces the interaction of selective protein complexes with the key structural protein SAP97, which tightly regulates the synaptic delivery of GluA1 and GluA4 AMPAR subunits. The results demonstrate that in the early stages of conditioning the initial activation of PKA stimulates the formation of a SAP97-AKAP/PKA-GluA1 protein complex leading to synaptic delivery of GluA1-containing AMPARs through a SAP97-PSD95 interaction. This is followed shortly thereafter by generation of a SAP97-KSR1/PKC-GluA4 complex for GluA4 AMPAR subunit delivery again through a SAP97-PSD95 interaction. These data suggest that SAP97 forms the molecular backbone of a protein scaffold critical for delivery of AMPARs to the PSD during conditioning. Together, the findings reveal a cooperative interaction of multiple scaffolding proteins for appropriately timed delivery of subunit-specific AMPARs to synapses and support a sequential two-stage model of AMPAR synaptic delivery during classical conditioning.

Neuroprotective effect of arctigenin via upregulation of P-CREB in mouse primary neurons and human SH-SY5Y neuroblastoma cells

Arctigenin (Arc) has been shown to act on scopolamine-induced memory deficit mice and to provide a neuroprotective effect on cultured cortical neurons from glutamate-induced neurodegeneration through mechanisms not completely defined. Here, we investigated the neuroprotective effect of Arc on H89-induced cell damage and its potential mechanisms in mouse cortical neurons and human SH-SY5Y neuroblastoma cells. We found that Arc prevented cell viability loss induced by H89 in human SH-SY5Y cells. Moreover, Arc reduced intracellular beta amyloid (Aβ) production induced by H89 in neurons and human SH-SY5Y cells, and Arc also inhibited the presenilin 1(PS1) protein level in neurons. In addition, neural apoptosis in both types of cells, inhibition of neurite outgrowth in human SH-SY5Y cells and reduction of synaptic marker synaptophysin (SYN) expression in neurons were also observed after H89 exposure. All these effects induced by H89 were markedly reversed by Arc treatment. Arc also significantly attenuated downregulation of the phosphorylation of CREB (p-CREB) induced by H89, which may contribute to the neuroprotective effects of Arc. These results demonstrated that Arc exerted the ability to protect neurons and SH-SY5Y cells against H89-induced cell injury via upregulation of p-CREB.

Identification of a functionally distinct truncated BDNF mRNA splice variant and protein in Trachemys scripta elegans

Brain-derived neurotrophic factor (BDNF) has a diverse functional role and complex pattern of gene expression. Alternative splicing of mRNA transcripts leads to further diversity of mRNAs and protein isoforms. Here, we describe the regulation of BDNF mRNA transcripts in an in vitro model of eyeblink classical conditioning and a unique transcript that forms a functionally distinct truncated BDNF protein isoform. Nine different mRNA transcripts from the BDNF gene of the pond turtle Trachemys scripta elegans (tBDNF) are selectively regulated during classical conditioning: exon I mRNA transcripts show no change, exon II transcripts are downregulated, while exon III transcripts are upregulated. One unique transcript that codes from exon II, tBDNF2a, contains a 40 base pair deletion in the protein coding exon that generates a truncated tBDNF protein. The truncated transcript and protein are expressed in the naïve untrained state and are fully repressed during conditioning when full-length mature tBDNF is expressed, thereby having an alternate pattern of expression in conditioning. Truncated BDNF is not restricted to turtles as a truncated mRNA splice variant has been described for the human BDNF gene. Further studies are required to determine the ubiquity of truncated BDNF alternative splice variants across species and the mechanisms of regulation and function of this newly recognized BDNF protein.

Two-stage AMPA receptor trafficking in classical conditioning and selective role for glutamate receptor subunit 4 (tGluA4) flop splice variant

Previously, we proposed a two-stage model for an in vitro neural correlate of eyeblink classical conditioning involving the initial synaptic incorporation of glutamate receptor A1 (GluA1)-containing α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid type receptors (AMPARs) followed by delivery of GluA4-containing AMPARs that support acquisition of conditioned responses. To test specific elements of our model for conditioning, selective knockdown of GluA4 AMPAR subunits was used using small-interfering RNAs (siRNAs). Recently, we sequenced and characterized the GluA4 subunit and its splice variants from pond turtles, Trachemys scripta elegans (tGluA4). Analysis of the relative abundance of mRNA expression by real-time RT-PCR showed that the flip/flop variants of tGluA4, tGluA4c, and a novel truncated variant tGluA4trc1 are major isoforms in the turtle brain. Here, transfection of in vitro brain stem preparations with anti-tGluA4 siRNA suppressed conditioning, tGluA4 mRNA and protein expression, and synaptic delivery of tGluA4-containing AMPARs but not tGluA1 subunits. Significantly, transfection of abducens motor neurons by nerve injections of tGluA4 flop rescue plasmid prior to anti-tGluA4 siRNA application restored conditioning and synaptic incorporation of tGluA4-containing AMPARs. In contrast, treatment with rescue plasmids for tGluA4 flip or tGluA4trc1 failed to rescue conditioning. Finally, treatment with a siRNA directed against GluA1 subunits inhibited conditioning and synaptic delivery of tGluA1-containing AMPARs and importantly, those containing tGluA4. These data strongly support our two-stage model of conditioning and our hypothesis that synaptic incorporation of tGluA4-containing AMPARs underlies the acquisition of in vitro classical conditioning. Furthermore, they suggest that tGluA4 flop may have a critical role in conditioning mechanisms compared with the other tGluA4 splice variants.

PKA increases in the olfactory bulb act as unconditioned stimuli and provide evidence for parallel memory systems: pairing odor with increased PKA creates intermediate- and long-term, but not short-term, memories

Neonatal odor-preference memory in rat pups is a well-defined associative mammalian memory model dependent on cAMP. Previous work from this laboratory demonstrates three phases of neonatal odor-preference memory: short-term (translation-independent), intermediate-term (translation-dependent), and long-term (transcription- and translation-dependent). Here, we use neonatal odor-preference learning to explore the role of olfactory bulb PKA in these three phases of mammalian memory. PKA activity increased normally in learning animals 10 min after a single training trial. Inhibition of PKA by Rp-cAMPs blocked intermediate-term and long-term memory, with no effect on short-term memory. PKA inhibition also prevented learning-associated CREB phosphorylation, a transcription factor implicated in long-term memory. When long-term memory was rescued through increased β-adrenoceptor activation, CREB phosphorylation was restored. Intermediate-term and long-term, but not short-term odor-preference memories were generated by pairing odor with direct PKA activation using intrabulbar Sp-cAMPs, which bypasses β-adrenoceptor activation. Higher levels of Sp-cAMPs enhanced memory by extending normal 24-h retention to 48-72 h. These results suggest that increased bulbar PKA is necessary and sufficient for the induction of intermediate-term and long-term odor-preference memory, and suggest that PKA activation levels also modulate memory duration. However, short-term memory appears to use molecular mechanisms other than the PKA/CREB pathway. These mechanisms, which are also recruited by β-adrenoceptor activation, must operate in parallel with PKA activation.

Rapid enrichment of presynaptic protein in boutons undergoing classical conditioning is mediated by brain-derived neurotrophic factor

Presynaptic structural modifications are thought to accompany activity-dependent synaptic plasticity and learning. This may involve the conversion of nonfunctional synapses into active ones or the generation of entirely new synapses. Here, using an in vitro neural analog of classical conditioning, we investigated presynaptic structural changes restricted to auditory nerve synapses that convey the conditioned stimulus (CS) by tract tracing using fluorescent tracers combined with immunostaining for the synaptic vesicle-associated protein synaptophysin. The results show that the size of presynaptic auditory boutons increased and the area and fluorescence intensity of punctate staining for synaptophysin were enhanced after conditioning. This occurred only for auditory nerve boutons apposed to the dendrites but not the somata of abducens motor neurons. Conditioning increased the percentage of boutons that were immunopositive for synaptophysin and enhanced the number of synaptophysin puncta they contained. Pretreatment with antibodies against brain-derived neurotrophic factor (BDNF) inhibited these conditioning-induced structural changes. There was also a net increase in the number of boutons apposed to abducens motor neurons after conditioning or BDNF treatment. These data indicate that the rapid enrichment of presynaptic boutons with proteins required for neurotransmitter recycling and release occurs during classical conditioning and that these processes are mediated by BDNF.

Transsynaptic EphB/Ephrin-B signaling regulates growth of presynaptic boutons required for classical conditioning

Learning-related presynaptic remodeling has been documented in only a few systems, and its molecular mechanisms are largely unknown. Here we describe a role for the bidirectional EphB/ephrin-B signaling system in structural plasticity of presynaptic nerve terminals using an in vitro model of classical conditioning. Conditioning or BDNF application induced significant growth of auditory nerve presynaptic boutons that convey the conditioned stimulus to abducens motor neurons. Interestingly, bouton enlargement occurred only for those synapses apposed to motor neuron dendrites rather than to somata. Phosphorylation of ephrin-B1, but not EphB2, was induced by both conditioning and BDNF application and was inhibited by postsynaptic injections of ephrin-B antibody. Finally, suppression of postsynaptic ephrin-B function inhibited presynaptic bouton enlargement that was rescued by activation of EphB2 by ephrin-B1-Fc. These data provide evidence for ephrin-B-induced EphB2 forward signaling in presynaptic structural plasticity during classical conditioning. They also reveal a functional interaction between BDNF/TrkB and the Eph/ephrin signaling systems in the coordination of presynaptic and postsynaptic modifications during conditioning.

Modeling signal transduction in classical conditioning with network motifs

Biological networks are constructed of repeated simplified patterns, or modules, called network motifs. Network motifs can be found in a variety of organisms including bacteria, plants, and animals, as well as intracellular transcription networks for gene expression and signal transduction processes in neuronal circuits. Standard models of signal transduction events for synaptic plasticity and learning often fail to capture the complexity and cooperativity of the molecular interactions underlying these processes. Here, we apply network motifs to a model for signal transduction during an in vitro form of eyeblink classical conditioning that reveals an underlying organization of these molecular pathways. Experimental evidence suggests there are two stages of synaptic AMPA receptor (AMPAR) trafficking during conditioning. Synaptic incorporation of GluR1-containing AMPARs occurs early to activate silent synapses conveying the auditory conditioned stimulus and this initial step is followed by delivery of GluR4 subunits that supports acquisition of learned conditioned responses (CRs). Overall, the network design of the two stages of synaptic AMPAR delivery during conditioning describes a coherent feed-forward loop (C1-FFL) with AND logic. The combined inputs of GluR1 synaptic delivery AND the sustained activation of 3-phosphoinositide-dependent protein-kinase-1 (PDK-1) results in synaptic incorporation of GluR4-containing AMPARs and the gradual acquisition of CRs. The network architecture described here for conditioning is postulated to act generally as a sign-sensitive delay element that is consistent with the non-linearity of the conditioning process. Interestingly, this FFL structure also performs coincidence detection. A motif-based approach to modeling signal transduction can be used as a new tool for understanding molecular mechanisms underlying synaptic plasticity and learning and for comparing findings across forms of learning and model systems.

Screening target specificity of siRNAs by rapid amplification of cDNA ends (RACE) for non-sequenced species

RNA interference (RNAi) is the process of sequence-specific posttranslational gene silencing triggered by double-stranded RNAs (dsRNAs). RNAi is a widely used approach for studying gene function. However, studies have shown that using siRNA can lead to off-target effects when the siRNA contains sufficient sequence identity to non-target mRNA sequences. One of the important steps in designing dsRNA is verification that it has sequence identity to only the target mRNA. In this report, we propose an approach for primary screening dsRNAs for potential off-target effects by using rapid amplification of cDNA ends. This method can be especially useful for model systems using species that have limited availability of sequence data.

Learning and memory consolidation: linking molecular and behavioral data

This paper puts together and links some classic and recent molecular data and hypothesis from different authors and laboratories related to learning and memory consolidation. Mainly addressed to non-specialists, it describes how the glutamatergic activation of plastic synapses in the hippocampus can give rise to new or enlarged dendritic spines which may constitute the main structural basis of some kind of memories. To establish learning and memory, the nervous system can use part of the same mechanisms which make the basic structure of neurons during the ontogenetic development of the brain. Through different families of kinases, phosphatases and other proteins, the activated N-methyl-d-aspartate (NMDA) receptors and different intracellular signals originated in the post-synaptic membranes can promote the synthesis of new proteins and the dynamic of actin. The consecutive morphological changes in the cytoskeleton of the neuron, later stabilized by new receptors inserted in the post-synaptic membranes, make possible memory consolidation. Short and long-term, as well as persistence, of memory mechanisms are related to these molecular processes. Recent research on system consolidation and memory allocation in neural circuits is also explained.

Oligomeric amyloid-{beta} inhibits the proteolytic conversion of brain-derived neurotrophic factor (BDNF), AMPA receptor trafficking, and classical conditioning

Amyloid-β (Aβ) peptide is thought to have a significant role in the progressive memory loss observed in patients with Alzheimer disease and inhibits synaptic plasticity in animal models of learning. We previously demonstrated that brain-derived neurotrophic factor (BDNF) is critical for synaptic AMPA receptor delivery in an in vitro model of eyeblink classical conditioning. Here, we report that acquisition of conditioned responses was significantly attenuated by bath application of oligomeric (200 nm), but not fibrillar, Aβ peptide. Western blotting revealed that BDNF protein expression during conditioning is significantly reduced by treatment with oligomeric Aβ, as were phosphorylation levels of cAMP-response element-binding protein (CREB), Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), Ca(2+)/calmodulin-dependent protein kinase IV (CaMKIV), and ERK. However, levels of PKA and PKCζ/λ were unaffected, as was PDK-1. Protein localization studies using confocal imaging indicate that oligomeric Aβ, but not fibrillar or scrambled forms, suppresses colocalization of GluR1 and GluR4 AMPA receptor subunits with synaptophysin, indicating that trafficking of these subunits to synapses during the conditioning procedure is blocked. In contrast, coapplication of BDNF with oligomeric Aβ significantly reversed these findings. Interestingly, a tolloid-like metalloproteinase in turtle, tTLLs (turtle tolloid-like protein), which normally processes the precursor proBDNF into mature BDNF, was found to degrade oligomeric Aβ into small fragments. These data suggest that an Aβ-induced reduction in BDNF, perhaps due to interference in the proteolytic conversion of proBDNF to BDNF, results in inhibition of synaptic AMPA receptor delivery and suppression of the acquisition of conditioning.

AMPA receptor trafficking and learning

In the last few years it has become clear that AMPA-type glutamate neurotransmitter receptors are rapidly transported into and out of synapses to strengthen or weaken their function. The remarkable dynamics of AMPA receptor (AMPAR) synaptic localization provides a compelling mechanism for understanding the cellular basis of learning and memory, as well as disease states involving cognitive dysfunction. Here, we summarize the evidence for AMPAR trafficking as a mechanism underlying a variety of learned responses derived from both behavioral and cellular studies. Evidence is also reviewed supporting synaptic dysfunction related to impaired AMPAR trafficking as a mechanism underlying learning and memory deficits in Alzheimer's disease. We conclude that emerging data support the concept of multistage AMPAR trafficking during learning and that a broad approach to include examination of all of the AMPAR subunits will provide a more complete view of the mechanisms underlying multiple forms of learning.

Cloning and characterization of glutamate receptor subunit 4 (GLUA4) and its alternatively spliced isoforms in turtle brain

Ionotropic glutamate receptors sensitive to α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), GluAs, play an important role in neural development, synaptic plasticity, and neurodegeneration. Previous studies using an in vitro model of eyeblink classical conditioning in pond turtles suggested that acquisition of conditioning is associated with synaptic delivery of AMPA receptors containing GluA4 subunits. However, sequences of the GluA4 subunit, expression profile, and its alternatively spliced isoforms in turtle brain have not been previously determined. The sequence and domain structure of turtle GluA4 (tGluA4) and its splice variants was characterized. We found ten isoforms of tGluA4 including several previously unidentified truncated variants. Analysis of the nucleotide sequences of tGluA4 flip/flop, tGluA4c flip/flop, and tGluA4s showed they are highly similar to known isoforms of the GluA4 subunit identified in chick. Examination of the relative abundance of mRNA expression for the tGluA4 variants showed that the flip and flop versions of tGluA4 and tGluA4c, and a novel truncated variant, tGluA4trc1, which is also expressed as protein, are major forms in the adult turtle brain. Identification of these alternatively spliced isoforms of tGluA4 will provide a unique opportunity to assess their role in synaptic plasticity through the application of short interfering RNAs.

Co-induction of LTP and LTD and its regulation by protein kinases and phosphatases

The cellular properties of long-term potentiation (LTP) following pairing of pre- and postsynaptic activity were examined at a known glutamatergic synapse in the leech, specifically between the pressure (P) mechanosensory and anterior pagoda (AP) neurons. Stimulation of the presynaptic P cell (25 Hz) concurrent with a 2 nA depolarization of the postsynaptic AP cell significantly potentiated the P-to-AP excitatory postsynaptic potential (EPSP) in an N-methyl-d-aspartate receptor (NMDAR)-dependent manner based on inhibitory effects of the NMDAR antagonist MK801 and inhibition of the NMDAR glycine binding site by 7-chlorokynurenic acid. LTP was blocked by injection of bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) into the postsynaptic (AP) cell, indicating a requirement for postsynaptic elevation of intracellular Ca(2+). Autocamtide-2-related inhibitory peptide (AIP), a specific inhibitor of Ca(2+)/calmodulin-dependent kinase II (CaMKII), and Rp-cAMP, an inhibitor of protein kinase A (PKA), also blocked pairing-induced potentiation, indicating a requirement for activation of CaMKII and PKA. Interestingly, application of AIP during pairing resulted in significantly depressed synaptic transmission. Co-application of AIP with the protein phosphatase inhibitor okadaic acid restored synaptic transmission to baseline levels, suggesting an interaction between CaMKII and protein phosphatases during induction of activity-dependent synaptic plasticity. When postsynaptic activity preceded presynaptic activity, NMDAR-dependent long-term depression (LTD) was observed that was blocked by okadaic acid. Postsynaptic injection of botulinum toxin blocked P-to-AP potentiation while postsynaptic injection of pep2-SVKI, an inhibitor of AMPA receptor endocytosis, inhibited LTD, supporting the hypothesis that glutamate receptor trafficking contributes to both LTP and LTD at the P-to-AP synapse in the leech.

Cleavage of proBDNF to BDNF by a tolloid-like metalloproteinase is required for acquisition of in vitro eyeblink classical conditioning

The tolloid/bone morphogenetic protein-1 family of metalloproteinases have an important role in the regulation of embryonic pattern formation and tissue morphogenesis. Studies suggest that they participate in mechanisms of synaptic plasticity in adults, but very little is known about their function. Recently, we isolated a reptilian ortholog of the tolloid gene family designated turtle tolloid-like gene (tTll). Here, we examined the role of tTLL in an in vitro model of eyeblink classical conditioning using an isolated brainstem preparation to assess its role in synaptic plasticity during conditioning. Analysis by real-time reverse transcription-PCR shows that an extracellularly secreted form of tTLL, tTLLs, is transiently expressed in the early stages of conditioning during conditioned response acquisition, whereas a cytosolic form, tTLLc, is not. Short interfering RNA (siRNA)-directed gene knockdown and rescue of tTLL expression demonstrate that it is required for conditioning. Significantly, we show that tTLLs cleaves the precursor proBDNF into mature BDNF in cleavage assay studies, and application of recombinant tTLLs protein alone to preparations results in induction of mature BDNF expression. The mature form of BDNF is minimally expressed in preparations treated with anti-tTLL siRNA, and the synaptic incorporation of both GluR1- and GluR4-containing AMPA receptors is significantly reduced, resulting in suppression of conditioning. This is the first study to demonstrate that expression of an extracellularly secreted tolloid-like metalloproteinase is regulated in the early stages of classical conditioning and functions in the conversion of proBDNF to mature BDNF. The mature form of BDNF is required for synaptic delivery of AMPA receptors and acquisition of conditioned responses.

Protein kinase C-dependent and independent signaling pathways regulate synaptic GluR1 and GluR4 AMPAR subunits during in vitro classical conditioning

Protein kinase C (PKC) signal transduction pathways have been implicated in mechanisms of synaptic plasticity and learning, however, the roles of the different PKC family isoforms remain to be clarified. Previous studies showed that NMDAR-mediated trafficking of GluR4-containing AMPARs supports conditioning and that the mitogen-activated protein kinases (MAPKs) have a central role in the synaptic delivery of GluR4 subunits. Here, an in vitro model of classical conditioning in pond turtles, Pseudemys scripta elegans, was used to assess the role of PKC isoforms in mechanisms underlying this form of learning. We show that the PKC antagonists chelerythrine and bisindolylmaleimide I attenuated conditioned response (CR) acquisition and expression, as did the PKCzeta pseudosubstrate peptide inhibitor ZIP. Analysis of protein expression revealed that PKCzeta is activated in early stages of conditioning followed shortly afterward by increased levels of PKCalpha/beta and activation of ERK MAPK. Data also suggest that PKCzeta is upstream from and activates ERK. Finally, protein localization studies using confocal imaging indicate that inhibitors of ERK, but not PKC, suppress colocalization of GluR1 with synaptophysin while inhibitors of PKC and ERK attenuate colocalization of GluR4 with synaptophysin. Together, these data suggest that acquisition of conditioning proceeds by two stages of AMPAR trafficking. The first is PKC-independent and ERK-dependent synaptic delivery of GluR1 subunits to activate silent synapses. This is followed by PKC-dependent and ERK-dependent synthesis and delivery of GluR4 subunits that supports the acquisition of CRs. Therefore, there is a selective role for PKC and MAPK signaling pathways in multistep AMPAR trafficking that mediates acquisition of classical conditioning.