A molecular switch for the consolidation of long-term memory: cAMP-inducible gene expression

Forskolin versus cAMP-Induced Decidualization and Survival of Endometrial Stromal Cells of Endometriosis Patients

Impairment of decidualization of eutopic human endometrial stromal cells (hESCs) may cause an increase in cell survival of endometrial tissue in the peritoneal cavity constituting a precondition for endometriosis development. Decidualization is a physiological process involving progesterone action and cAMP signaling. We here evaluated the effect of 8-Br-cAMP, the adenylate cyclase activator forskolin and of the progestin progesterone and medroxyprogesterone acetate (MPA) alone and in combination on decidualization induction using prolactin ELISA, and on cell size, cell granularity, and cell survival via flow cytometry in hESCs of patients with and without endometriosis. While progestins alone did not induce functional decidualization in hESCs, 8-Br-cAMP and forskolin induced decidualization in hESCs from both cohorts, whereas the induction of FOXO1 transcription and prolactin secretion by forskolin was significantly lower than by 8-Br-cAMP. 8-Br-cAMP- and forskolin-induced prolactin secretion was significantly enhanced by MPA, but not by progesterone. Decidualization entailed a decrease in cell size and in cell granularity. In general, hESCs from women with mild (ASRM I/II) as well as severe (ASRM III/IV) endometriosis in trend displayed a higher granularity, whereas mainly hESCs from severe endometriosis showed a stronger resistance to the induction of cell death after decidualization induction. In both cohorts, the amount of the decidual marker protein prolactin rather exhibited an anti-proportional correlation to cell death induction during six day treatment. This study contributes to widen our understanding of the connection of decidualization and cell death in endometriosis.

Establishment and characterization of a new class of adenylate cyclases (class VII ACs) in plants

Adenylate cyclase is the key enzyme in the synthesis of cAMP. Now, more and more plant genes which possessing AC function are being identified, but the classification of plant ACs has not yet been systematically studied and the relationship of plant ACs with other existing six classes ACs in animals and microorganisms is still unclear. In this study, we found that 7 of the 15 reported plant ACs with conserved CYTH-like_AC_Ⅳ-like domain were clustered into a group with high confidence (Group Ⅳ), while the other plant ACs were clustered into other three groups with no common domain. In addition, we also found that the Group Ⅳ plant ACs were grouped into an independent and specific class (Class VII), separated from the existing six classes of ACs. The Group Ⅳ plant ACs, compared to the existing six classes of ACs, own unique CYTH-like_AC_Ⅳ-like conserved domain and EXEXK signature motif, characteristic protein tertiary structures, specific subcellular localization and catalytic conditions. In view of the above, we regarded the Group Ⅳ plant ACs as the seventh class of AC (VII AC). This study does the systematic classification of plant ACs which could lay a foundation for further identification and study of the biological functions of the plant-specific VII ACs.

A triphosphate tunnel metalloenzyme from pear (PbrTTM1) moonlights as an adenylate cyclase

Adenylyl cyclase (AC) is the vital enzyme for generating 3',5'-cyclic adenosine monophosphate, an important signaling molecule with profound nutritional and medicinal values. However, merely, a dozen of AC proteins have been reported in plants so far. Here, a protein annotated as triphosphate tunnel metalloenzyme (PbrTTM1) in pear, the important worldwide fruit plant, was firstly identified to possess AC activity with both in vivo and in vitro methods. It exhibited a relatively low AC activity but was capable of complementing AC functional deficiencies in the E. coli SP850 strain. Its protein conformation and potential catalytic mechanism were analyzed by means of biocomputing. The active site of PbrTTM1 is a closed tunnel constructed by nine antiparallel β-folds surrounded with seven helices. Inside the tunnel, the charged residues were possibly involved in the catalytic process by coordinating with divalent cation and ligand. The hydrolysis activity of PbrTTM1 was tested as well. Compared to the much higher capacity of hydrolyzing, the AC activity of PbrTTM1 tends to be a moonlight function. Through a comparison of protein structures in various plant TTMs, it is reasonable to speculate that many plant TTMs might possess AC activity as a form of moonlighting enzyme function.

Three Novel Adenylate Cyclase Genes Show Significant Biological Functions in Plant

Adenylate cyclase is the key enzyme solely synthesizing cAMP which participates in cell metabolism regulations and functions as an intracellular second messenger. However, the biological functions of plant ACs have not been elucidated clearly for their poor conservative sequences and low detectable cAMP. We performed a systematic study of plant ACs by using Chinese jujube, whose fruit exhibits the highest cAMP content among plants. Three novel ACs were identified from Chinese jujube, and two types of methods including in vitro and in vivo were used to certificate ZjAC1-3 which can catalyze the conversion of ATP into cAMP. The biological functions of significant accelerations of seed germination, root growth, and flowering were found via overexpression of these AC genes in Arabidopsis, and these functions of ACs were further demonstrated by treating the AC-overexpressing transgenic lines and wild type Arabidopsis with bithionol and dibutyryl-cAMP. At last, transcriptome data revealed that the underlying mechanism of the biological functions of ACs might be regulation of the key genes involved in the circadian rhythm pathway and the hormone signal transduction pathway. This research established a foundation for further investigating plant AC genes and provided strong evidence for cAMP serving as a signaling molecule in plants.

State-selective frustration as a key driver of allosteric pluripotency

Allosteric pluripotency arises when an allosteric effector switches from agonist to antagonist depending on the experimental conditions. For example, the Rp-cAMPS ligand of Protein Kinase A (PKA) switches from agonist to antagonist as the MgATP concentration increases and/or the kinase substrate affinity or concentration decreases. Understanding allosteric pluripotency is essential to design effective allosteric therapeutics with minimal side effects. Allosteric pluripotency of PKA arises from divergent allosteric responses of two homologous tandem cAMP-binding domains, resulting in a free energy landscape for the Rp-cAMPS-bound PKA regulatory subunit R1a in which the ground state is kinase inhibition-incompetent and the kinase inhibition-competent state is excited. The magnitude of the free energy difference between the ground non-inhibitory and excited inhibitory states (ΔG_R,Gap) relative to the effective free energy of R1a binding to the catalytic subunit of PKA (ΔG_R:C) dictates whether the antagonism-to-agonism switch occurs. However, the key drivers of ΔG_R,Gap are not fully understood. Here, by analyzing an R1a mutant that selectively silences allosteric pluripotency, we show that a major determinant of ΔG_R,Gap unexpectedly arises from state-selective frustration in the ground inhibition-incompetent state of Rp-cAMPS-bound R1a. Such frustration is caused by steric clashes between the phosphate-binding cassette and the helices preceding the lid, which interact with the phosphate and base of Rp-cAMPS, respectively. These clashes are absent in the excited inhibitory state, thus reducing the ΔG_R,Gap to values comparable to ΔG_R:C, as needed for allosteric pluripotency to occur. The resulting model of allosteric pluripotency is anticipated to assist the design of effective allosteric modulators.

The Rp-cAMPS ligand of protein kinase A switches from agonist to antagonist depending on metabolite and proteomic contexts. We show that the state-selective frustration is a key driver of this allosteric pluripotency phenomenon.

Memory Function in Feeding Habit Transformation of Mandarin Fish (Siniperca chuatsi)

Mandarin fish refuse dead prey fish or artificial diets and can be trained to transform their inborn feeding habit. To investigate the effect of memory on feeding habit transformation, we compared the reaction time to dead prey fish and the success rate of feeding habit transformation to dead prey fish with training of mandarin fish in the 1st experimental group (trained once) and the 2nd experimental group (trained twice). The mandarin fish in the 2nd group had higher success rate of feeding habit transformation (100%) than those in the 1st group (67%), and shorter reaction time to dead prey fish (<1 s) than those in the 1st group (>1 s). Gene expression of cAMP responsive element binding protein I (Creb I), brain-derived neurotrophic factor (Bdnf), CCAAT enhancer binding protein delta (C/EBPD), fos-related antigen 2 (Fra2), and proto-oncogenes c-fos (c-fos) involved in long-term memory formation were significantly increased in the 2nd group after repeated training, and taste 1 receptor member 1 (T1R1), involved in feeding habit formation, was significantly increased in brains of the 2nd group after repeated training. DNA methylation levels at five candidate CpG (cytosine–guanine) sites contained in the predicted CpG island in the 5′-flanking region of T1R1 were significantly decreased in brains of the 2nd group compared with that of the 1st group. These results indicated that the repeated training can improve the feeding habit transformation through the memory formation of accepting dead prey fish. DNA methylation of the T1R1 might be a regulatory factor for feeding habit transformation from live prey fish to dead prey fish in mandarin fish.

Diminished CRE-Induced Plasticity is Linked to Memory Deficits in Familial Alzheimer's Disease Mice

The mechanism underlying impaired learning and memory in Alzheimer's disease is not fully elucidated. The phosphorylation of cyclic-AMP response element binding protein (pCREB) in the hippocampus is thought to be a critical initiating step in the formation of long-term memories. Here, we tested CRE-driven gene expression following learning in mice harboring the familial Alzheimer's disease-linked APPswe/PS1ΔE9 mutations using CRE-β galactosidase reporter. We show that young adult APPswe/PS1ΔE9 mice exhibit impaired recognition memory and reduced levels of pCREB, and its cofactors CREB binding protein (CBP) and p-300 following a learning task, compared to their wild type littermate counterparts. Impairments in learning-induced activation of CREB in these mice are manifested by reduced CRE-driven gene transcription. Importantly, expression of the CRE-driven immediate early gene, Egr-1 (Zif268) is decreased in the CA1 region of the hippocampus. These studies implicate defective CREB-dependent plasticity in the mechanism underlying learning and memory deficits in Alzheimer's disease.

Conserved regulators of cognitive aging: From worms to humans

Cognitive decline is a major deficit that arises with age in humans. While some research on the underlying causes of these problems can be done in humans, harnessing the strengths of small model systems, particularly those with well-studied longevity mutants, such as the nematode C. elegans, will accelerate progress. Here we review the approaches being used to study cognitive decline in model organisms and show how simple model systems allow the rapid discovery of conserved molecular mechanisms, which will eventually enable the development of therapeutics to slow cognitive aging.

Tapping the translation potential of cAMP signalling: molecular basis for selectivity in cAMP agonism and antagonism as revealed by NMR

Eukaryotic CBDs (cAMP-binding domains) control multiple cellular functions (e.g. phosphorylation, guanine exchange and ion channel gating). Hence the manipulation of cAMP-dependent signalling pathways has a high translational potential. However, the ubiquity of eukaryotic CBDs also poses a challenge in terms of selectivity. Before the full translational potential of cAMP signalling can be tapped, it is critical to understand the structural basis for selective cAMP agonism and antagonism. Recent NMR investigations have shown that structurally homologous CBDs respond differently to several CBD ligands and that these unexpected differences arise at the level of either binding (i.e. affinity) or allostery (i.e. modulation of the autoinhibitory equilibria). In the present article, we specifically address how the highly conserved CBD fold binds cAMP with markedly different affinities in PKA (protein kinase A) relative to other eukaryotic cAMP receptors, such as Epac (exchange protein directly activated by cAMP) and HCN (hyperpolarization-activated cyclic-nucleotide-modulated channel). A major emerging determinant of cAMP affinity is hypothesized to be the position of the autoinhibitory equilibrium of the apo-CBD, which appears to vary significantly across different CBDs. These analyses may assist the development of selective CBD effectors that serve as potential drug leads for the treatment of cardiovascular diseases.

Epigenetic mechanisms and memory strength: a comparative study

Memory consolidation requires de novo mRNA and protein synthesis. Transcriptional activation is controlled by transcription factors, their cofactors and repressors. Cofactors and repressors regulate gene expression by interacting with basal transcription machinery, remodeling chromatin structure and/or chemically modifying histones. Acetylation is the most studied epigenetic mechanism of histones modifications related to gene expression. This process is regulated by histone acetylases (HATs) and histone deacetylases (HDACs). More than 5 years ago, we began a line of research about the role of histone acetylation during memory consolidation. Here we review our work, presenting evidence about the critical role of this epigenetic mechanism during consolidation of context-signal memory in the crab Neohelice granulata, as well as during consolidation of novel object recognition memory in the mouse Mus musculus. Our evidence demonstrates that histone acetylation is a key mechanism in memory consolidation, functioning as a distinctive molecular feature of strong memories. Furthermore, we found that the strength of a memory can be characterized by its persistence or its resistance to extinction. Besides, we found that the role of this epigenetic mechanism regulating gene expression only in the formation of strongest memories is evolutionarily conserved.

Mechanisms underlying early odor preference learning in rats

Early odor preference training in rat pups produces behavioral preferences that last from hours to lifetimes. Here, we discuss the molecular and circuitry changes we have observed in the olfactory bulb (OB) and in the anterior piriform cortex (aPC) following odor training. For normal preference learning, both structures are necessary, but learned behavior can be initiated by initiating local circuit change in either structure. Our evidence relates dynamic molecular and circuit changes to memory duration and storage localization. Results using this developmental model are consistent with biological memory theories implicating N-methyl-D-aspartate (NMDA) receptors and β-adrenoceptors, and their associated cascades, in memory induction and consolidation. Finally, our examination of the odor preference model reveals a primary role for increases in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor synaptic strength, and in network strength, in the creation and maintenance of preference memory in both olfactory structures.

Targeting protein-protein interactions within the cyclic AMP signaling system as a therapeutic strategy for cardiovascular disease

The cAMP signaling system can trigger precise physiological cellular responses that depend on the fidelity of many protein-protein interactions, which act to bring together signaling intermediates at defined locations within cells. In the heart, cAMP participates in the fine control of excitation-contraction coupling, hence, any disregulation of this signaling cascade can lead to cardiac disease. Due to the ubiquitous nature of the cAMP pathway, general inhibitors of cAMP signaling proteins such as PKA, EPAC and PDEs would act non-specifically and universally, increasing the likelihood of serious 'off target' effects. Recent advances in the discovery of peptides and small molecules that disrupt the protein-protein interactions that underpin cellular targeting of cAMP signaling proteins are described and discussed.

CCAAT enhancer binding protein δ plays an essential role in memory consolidation and reconsolidation

A newly formed memory is temporarily fragile and becomes stable through a process known as consolidation. Stable memories may again become fragile if retrieved or reactivated, and undergo a process of reconsolidation to persist and strengthen. Both consolidation and reconsolidation require an initial phase of transcription and translation that lasts for several hours. The identification of the critical players of this gene expression is key for understanding long-term memory formation and persistence. In rats, the consolidation of inhibitory avoidance (IA) memory requires gene expression in both the hippocampus and amygdala, two brain regions that process contextual/spatial and emotional information, respectively; IA reconsolidation requires de novo gene expression in the amygdala. Here we report that, after IA learning, the levels of the transcription factor CCAAT enhancer binding protein δ (C/EBPδ) are significantly increased in both the hippocampus and amygdala. These increases are essential for long-term memory consolidation, as their blockade via antisense oligodeoxynucleotide-mediated knockdown leads to memory impairment. Furthermore, C/EBPδ is upregulated and required in the amygdala for IA memory reconsolidation. C/EBPδ is found in nuclear, somatic, and dendritic compartments, and a dendritic localization of C/EBPδ mRNA in hippocampal neuronal cultures suggests that this transcription factor may be translated at synapses. Finally, the induction of long-term potentiation at CA3-CA1 synapses by tetanic stimuli in acute slices, a cellular model of long-term memory, leads to an accumulation of C/EBPδ in the nucleus. We conclude that the transcription factor C/EBPδ plays a critical role in memory consolidation and reconsolidation.

AU-rich element-binding protein negatively regulates CCAAT enhancer-binding protein mRNA stability during long-term synaptic plasticity in Aplysia

The consolidation of long-term memory for sensitization and synaptic facilitation in Aplysia requires synthesis of new mRNA including the immediate early gene Aplysia CCAAT enhancer-binding protein (ApC/EBP). After the rapid induction of ApC/EBP expression in response to repeated treatments of 5-hydroxytryptamine (5-HT), ApC/EBP mRNA is temporarily expressed in sensory neurons of sensory-to-motor synapses. However, the molecular mechanism underlying the rapid degradation of ApC/EBP transcript is not known. Here, we cloned an AU-rich element (ARE)-binding protein, ApAUF1, which functions as a destabilizing factor for ApC/EBP mRNA. ApAUF1 was found to bind to the 3' UTR of ApC/EBP mRNA that contains AREs and subsequently reduces the expression of ApC/EBP 3' UTR-containing reporter genes. Moreover, overexpression of ApAUF1 inhibited the induction of ApC/EBP mRNA in sensory neurons and also impaired long-term facilitation of sensory-to-motor synapses by repetitive 5-HT treatments. These results provide evidence for a critical role of the posttranscriptional modification of ApC/EBP mRNA during the consolidation of synaptic plasticity.

Environmental regulation of the neural epigenome

There are numerous examples of enduring effects of early experience on gene transcription and neural function. We review the emerging evidence for epigenetics as a candidate mechanism for such effects. There is now evidence that intracellular signals activated by environmental events can directly modify the epigenetic state of the genome, including CpG methylation, histone modifications and microRNAs. We suggest that this process reflects an activity-dependent epigenetic plasticity at the level of the genome, comparable with that observed at the synapse. This epigenetic plasticity mediates neuronal differentiation and phenotypic plasticity, including that associated with learning and memory. Altered epigenetic states are also associated with the risk for and expression of mental disorders. In a broader context, these studies define a biological basis for the interplay between environmental signals and the genome in the regulation of individual differences in behavior, cognition and physiology, as well as the risk for psychopathology.

Stimulation-dependent intraspinal microtubules and synaptic failure in Alzheimer's disease: a review

There are many microtubules in axons and dendritic shafts, but it has been thought that there were fewer microtubules in spines. Recently, there have been four reports that observed the intraspinal microtubules. Because microtubules originate from the centrosome, these four reports strongly suggest a stimulation-dependent connection between the nucleus and the stimulated postsynaptic membrane by microtubules. In contrast, several pieces of evidence suggest that spine elongation may be caused by the polymerization of intraspinal microtubules. This structural mechanism for spine elongation suggests, conversely, that the synapse loss or spine loss observed in Alzheimer's disease may be caused by the depolymerization of intraspinal microtubules. Based on this evidence, it is suggested that the impairment of intraspinal microtubules may cause spinal structural change and block the translocation of plasticity-related molecules between the stimulated postsynaptic membranes and the nucleus, resulting in the cognitive deficits of Alzheimer's disease.

DISC1: Structure, Function, and Therapeutic Potential for Major Mental Illness

Disrupted in schizophrenia 1 (DISC1) is well established as a genetic risk factor across a spectrum of psychiatric disorders, a role supported by a growing body of biological studies, making the DISC1 protein interaction network an attractive therapeutic target. By contrast, there is a relative deficit of structural information to relate to the myriad biological functions of DISC1. Here, we critically appraise the available bioinformatics and biochemical analyses on DISC1 and key interacting proteins, and integrate this with the genetic and biological data. We review, analyze, and make predictions regarding the secondary structure and propensity for disordered regions within DISC1, its protein-interaction domains, subcellular localization motifs, and the structural and functional implications of common and ultrarare DISC1 variants associated with major mental illness. We discuss signaling pathways of high pharmacological potential wherein DISC1 participates, including those involving phosphodiesterase 4 (PDE4) and glycogen synthase kinase 3 (GSK3). These predictions and priority areas can inform future research in the translational and potentially guide the therapeutic processes.

Presynaptically silent synapses: dormancy and awakening of presynaptic vesicle release

Synapses represent the main junctures of communication between neurons in the nervous system. In many neurotransmitter systems, a fraction of presynaptic terminals fails to release vesicles in response to action potential stimulation and strong calcium influx. These silent presynaptic terminals exhibit a reversible functional dormancy beyond low vesicle release probability, and dormancy status may have important implications in neural function. Recent advances have implicated presynaptic proteins interacting with vesicles downstream of cAMP and protein kinase A signaling cascades in modulating the number of these mute presynaptic terminals, and dormancy induction may represent a homeostatic neuroprotective mechanism active during pathological insults involving excitotoxicity. Interestingly, dormancy reversal may also be induced during Hebbian plasticity. Here, details of synaptic dormancy, recent insights into the molecular signaling cascades involved, and potential clinical and mechanistic implications of this form of synaptic plasticity are described.

Persistent long-term synaptic plasticity requires activation of a new signaling pathway by additional stimuli

Most memories are strengthened by additional stimuli, but it is unclear how additional stimulation or training reinforces long-term memory. To address this we examined whether long-term facilitation (LTF) of Aplysia sensorimotor synapses in cell culture-a cellular correlate of long-term sensitization of defensive withdrawal reflexes in Aplysia californica-can be prolonged by additional stimulation. We found that 1 d treatment with serotonin (5-HT; five brief applications at 20 min intervals) produced LTF lasting ∼3 d, whereas 2 d of such 5-HT treatments induced a persistent LTF lasting >7 d. Incubation with the protein synthesis inhibitor rapamycin during the second set of 5-HT treatments abolished all facilitation, and synapse strength returned prematurely to baseline. Persistent LTF required more persistent elevation in the expression of the neurotrophin-like peptide sensorin and its secretion. Activation of protein kinase C (PKC) during the second day of 5-HT treatments, not required for LTF or changes in sensorin expression during the first set of 5-HT treatments, is critical for persistent LTF and replaces phosphoinositide 3 kinase (PI3K) activity in mediating the increase in sensorin expression. In contrast, activations of PKC during the first day of 5-HT treatments and PI3K during the second day of 5-HT treatments are unnecessary for persistent LTF or the increases in sensorin expression. Thus, additional stimuli make preexisting plasticity labile as they recruit a new signaling cascade to regulate the synthesis of a neurotrophin-like peptide required for persistent alterations in synaptic efficacy.

Mechanisms of induction and maintenance of spike-timing dependent plasticity in biophysical synapse models

We review biophysical models of synaptic plasticity, with a focus on spike-timing dependent plasticity (STDP). The common property of the discussed models is that synaptic changes depend on the dynamics of the intracellular calcium concentration, which itself depends on pre- and postsynaptic activity. We start by discussing simple models in which plasticity changes are based directly on calcium amplitude and dynamics. We then consider models in which dynamic intracellular signaling cascades form the link between the calcium dynamics and the plasticity changes. Both mechanisms of induction of STDP (through the ability of pre/postsynaptic spikes to evoke changes in the state of the synapse) and of maintenance of the evoked changes (through bistability) are discussed.

Epigenetics and the biological basis of gene x environment interactions

OBJECTIVE

Child and adolescent psychiatry is rife with examples of the sustained effects of early experience on brain function. The study of behavioral genetics provides evidence for a relation between genomic variation and personality and with the risk for psychopathology. A pressing challenge is that of conceptually integrating findings from genetics into the study of personality without regressing to arguments concerning the relative importance of genomic variation versus nongenomic or environmental influences.

METHOD

Epigenetics refers to functionally relevant modifications to the genome that do not involve a change in nucleotide sequence. This review examines epigenetics as a candidate biological mechanism for gene x environment interactions, with a focus on environmental influences that occur during early life and that yield sustained effects on neural development and function.

RESULTS

The studies reviewed suggest that epigenetic remodeling occurs in response to the environmental activation of cellular signalling pathways associated with synaptic plasticity, epigenetic marks are actively remodeled during early development in response to environmental events that regulate neural development and function, and epigenetic marks are subject to remodeling by environmental influences even at later stages in development.

CONCLUSION

Epigenetic remodeling might serve as an ideal mechanism for phenotypic plasticity--the process whereby the environment interacts with the genome to produce individual differences in the expression of specific traits.

Epigenetics and the environmental regulation of the genome and its function

There are numerous examples in psychology and other disciplines of the enduring effects of early experience on neural function. In this article, we review the emerging evidence for epigenetics as a candidate mechanism for these effects. Epigenetics refers to functionally relevant modifications to the genome that do not involve a change in nucleotide sequence. Such modifications include chemical marks that regulate the transcription of the genome. There is now evidence that environmental events can directly modify the epigenetic state of the genome. Thus studies with rodent models suggest that during both early development and in adult life, environmental signals can activate intracellular pathways that directly remodel the "epigenome," leading to changes in gene expression and neural function. These studies define a biological basis for the interplay between environmental signals and the genome in the regulation of individual differences in behavior, cognition, and physiology.

Gastric secretion elicited by conditioning in rats

OBJECTIVE

To investigate whether interdigestive gastric acid secretion can be controlled by a possible memory-related cortical mechanism.

MATERIAL AND METHODS

To evaluate gastric secretion in rats, we used a methodology that allows gastric juice collection in rats in their habitual conditions (without any restraining) by pairing sound as the conditioning stimulus (CS) and food as the unconditioning stimulus (US). The levels of gastric acid secretion under basal conditions and under sound stimulation were recorded and the circulating gastrin levels determined.

RESULTS

When the gastric juice was collected in the course of the conditioning procedure, the results showed that under noise stimulation a significant increase in gastric acid secretion occurred after 10 days of conditioning (p<0.01). The significance was definitively demonstrated after 13 days of conditioning (p<0.001). Basal secretions of the conditioned rats reached a significant level after 16 days of conditioning. The levels of noise-stimulated gastric acid secretion were the highest so far described in physiological experiments carried out in rats and there were no significant increases in the circulating gastrin levels.

CONCLUSIONS

The results point to the important role played by cortical structures in the control of interdigestive gastric acid secretion in rats. If this mechanism is also present in humans, it may be involved in diseases caused by inappropriate gastric acid secretion during the interprandial periods.

Systemic administration of rolipram increases medullary and spinal cAMP and activates a latent respiratory motor pathway after high cervical spinal cord injury

BACKGROUND/OBJECTIVE

High cervical spinal cord hemisection interrupts descending respiratory drive from the rostral ventral respiratory group in the medulla to the ipsilateral phrenic motoneurons. Hemisection results in the paralysis of the ipsilateral hemidiaphragm. Chronic administration of rolipram, a specific phosphodiesterase-IV inhibitor, promotes synaptic plasticity and restores phrenic nerve function after a high cervical spinal cord lesion. Here, we test the hypothesis that an acute administration of rolipram will increase spinal and medullary levels of 3',5'-cyclic adenosine monophosphate (cAMP) and induce phrenic nerve recovery after cervical (C2) spinal cord hemisection.

METHODS

Male Sprague-Dawley rats were subjected to left C2 hemisection surgery 1 week before experimentation. Bilateral phrenic nerve activity was recorded in anesthetized, vagotomized, and pancuronium paralyzed rats, and rolipram was intravenously applied (2 mg/kg).

RESULTS

Intravenous administration of rolipram increased phrenic nerve output in uninjured control and left C2 spinal cord-hemisected rats. In addition, rolipram restored respiratory-related activity to the left phrenic nerve made quiescent by the hemisection. In both uninjured and hemisected rats, rolipram significantly enhanced phrenic inspiratory burst amplitude and burst area compared with predrug values. Also, rolipram concomitantly increased spinal and medullary cAMP.

CONCLUSIONS

These results suggest that a phosphodiesterase inhibitor capable of elevating cAMP levels can enhance phrenic nerve output and restore respiratory-related phrenic nerve function after high cervical spinal cord injury. Thus, targeting the cAMP signaling cascade can be a useful therapeutic approach in promoting synaptic efficacy and respiratory recovery after cervical spinal cord injury.

Transcription factors in long-term memory and synaptic plasticity

Transcription is a molecular requisite for long-term synaptic plasticity and long-term memory formation. Thus, in the last several years, one main interest of molecular neuroscience has been the identification of families of transcription factors that are involved in both of these processes. Transcription is a highly regulated process that involves the combined interaction and function of chromatin and many other proteins, some of which are essential for the basal process of transcription, while others control the selective activation or repression of specific genes. These regulated interactions ultimately allow a sophisticated response to multiple environmental conditions, as well as control of spatial and temporal differences in gene expression. Evidence based on correlative changes in expression, genetic mutations, and targeted molecular inhibition of gene expression have shed light on the function of transcription in both synaptic plasticity and memory formation. This review provides a brief overview of experimental work showing that several families of transcription factors, including CREB, C/EBP, Egr, AP-1, and Rel, have essential functions in both processes. The results of this work suggest that patterns of transcription regulation represent the molecular signatures of long-term synaptic changes and memory formation.

Transcriptional regulation of long-term memory in the marine snail Aplysia

Whereas the induction of short-term memory involves only covalent modifications of constitutively expressed preexisting proteins, the formation of long-term memory requires gene expression, new RNA, and new protein synthesis. On the cellular level, transcriptional regulation is thought to be the starting point for a series of molecular steps necessary for both the initiation and maintenance of long-term synaptic facilitation (LTF). The core molecular features of transcriptional regulation involved in the long-term process are evolutionally conserved in Aplysia, Drosophila, and mouse, and indicate that gene regulation by the cyclic AMP response element binding protein (CREB) acting in conjunction with different combinations of transcriptional factors is critical for the expression of many forms of long-term memory. In the marine snail Aplysia, the molecular mechanisms that underlie the storage of long-term memory have been extensively studied in the monosynaptic connections between identified sensory neuron and motor neurons of the gill-withdrawal reflex. One tail shock or one pulse of serotonin (5-HT), a modulatory transmitter released by tail shocks, produces a transient facilitation mediated by the cAMP-dependent protein kinase leading to covalent modifications in the sensory neurons that results in an enhancement of transmitter release and a strengthening of synaptic connections lasting minutes. By contrast, repeated pulses of 5-hydroxytryptamine (5-HT) induce a transcription- and translation-dependent long-term facilitation (LTF) lasting more than 24 h and trigger the activation of a family of transcription factors in the presynaptic sensory neurons including ApCREB1, ApCREB2 and ApC/EBP. In addition, we have recently identified novel transcription factors that modulate the expression of ApC/EBP and also are critically involved in LTF. In this review, we examine the roles of these transcription factors during consolidation of LTF induced by different stimulation paradigms.

Administration of phosphodiesterase inhibitors and an adenosine A1 receptor antagonist induces phrenic nerve recovery in high cervical spinal cord injured rats

High cervical spinal cord hemisection interrupts the descending respiratory drive from the medulla to the ipsilateral phrenic motoneurons, consequently leading to the paralysis of the ipsilateral hemidiaphragm. Previous studies have shown that chronic oral administration of theophylline, a phosphodiesterase inhibitor and an adenosine receptor antagonist, can restore function to the quiescent phrenic nerve and hemidiaphragm ipsilateral to hemisection. Both of these actions of theophylline result in an increase in 3'-5'-cyclic adenosine monophosphate (cAMP). Furthermore, the chronic theophylline-mediated respiratory recovery persists long after the animals have been weaned from the drug. To date, the precise cellular mechanisms underlying the recovery induced by theophylline are still not known. Since theophylline has two modes of action, in the present study we tested whether chronic administration of pentoxifylline, a non-selective phosphodiesterase inhibitor, rolipram, a phosphodiesterase-4 specific inhibitor, and 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), an adenosine A1 receptor antagonist, would induce recovery similar to that induced by theophylline in male Sprague-Dawley rats following a left C2 spinal cord lesion. Recovery of left phrenic nerve activity was assessed at 5 or 10 days after the last drug administrations to assess the persistent nature of the recovery. Pentoxifylline, rolipram and DPCPX, all capable of modulating 3',5'-cyclic monophosphate (cAMP) levels, brought about long-term respiratory recovery in the phrenic nerve ipsilateral to the left C2 lesion at 5 and 10 days after the last drug administration. Therefore, these results suggest that compounds capable of regulating cAMP levels may be therapeutically useful in promoting functional recovery following spinal cord injury.

Functional genomics and schizophrenia: endophenotypes and mutant models

This article summarizes the rationale, methods, and results of gene discovery programs in schizophrenia research and describes functional methods of investigating potential candidate genes. It focuses next on the most prominent current candidate genes and describes (1) evidence for their association with schizophrenia and research into the function of each gene; (2) investigation of the clinical phenotypes and endophenotypes associated with each gene, at the levels of psychopathologic, neurocognitive, electrophysiologic, neuroimaging, and neuropathologic findings; and (3) research into the ethologic, cognitive, social, and psychopharmacologic phenotype of mutants with targeted deletion of each gene. It examines gene-gene and gene-environment interactions. Finally, it looks at future directions for research.

Rethinking the fear circuit: the central nucleus of the amygdala is required for the acquisition, consolidation, and expression of Pavlovian fear conditioning

In the standard model of pavlovian fear learning, sensory input from neutral and aversive stimuli converge in the lateral nucleus of the amygdala (LA), in which alterations in synaptic transmission encode the association. During fear expression, the LA is thought to engage the central nucleus of the amygdala (CE), which serves as the principal output nucleus for the expression of conditioned fear responses. In the present study, we reexamined the roles of LA and CE. Specifically, we asked whether CE, like LA, might also be involved in fear learning and memory consolidation. Using functional inactivation methods, we first show that CE is involved not only in the expression but also the acquisition of fear conditioning. Next, we show that inhibition of protein synthesis in CE after training impairs fear memory consolidation. These findings indicate that CE is not only involved in fear expression but, like LA, is also involved in the learning and consolidation of pavlovian fear conditioning.

Disrupted in schizophrenia 1: building brains and memories

Schizophrenia and bipolar affective disorder are common, debilitating, and poorly understood and treated disorders. Both conditions are highly heritable. Recent genetic studies have suggested that the gene disrupted in schizophrenia 1 (DISC1) is an important risk factor. DISC1 seems to have a key role in building the brain and memories by interacting with other proteins, including nuclear distribution E-like protein and phosphodiesterase 4B. Here, we review the current knowledge, highlight some key unanswered questions and propose ways forward towards a better understanding of normal and abnormal brain development and function. In the long term, this might lead to the discovery of drugs that are more efficacious and safer than currently available ones.

Use of c-fos to identify activity-dependent spinal neurons after stepping in intact adult rats

STUDY DESIGN

An investigation of c-fos activation pattern in spinal neurons of intact adult rats after acute bouts of treadmill locomotion.

OBJECTIVES

To map spinal neurons that are involved in quadrupedal treadmill stepping of intact adult rats by using c-fos as a marker.

SETTINGS

Los Angeles, CA, USA.

METHODS

Spinal cord sections of rats that were not stepped (n = 4) were used to map the FOS-positive (+) neurons under basal conditions. The stepped group (n = 16) was placed on a treadmill to step quadrupedally for varying durations to induce c-fos activity. Spinal cord sections of thoracic and lumbar segments of Stp and Nstp rats were processed using a c-fos antibody, choline acetyl transferase and heat shock protein 27 for identifying motoneurons.

RESULTS

Stepping induced a greater number of FOS+ neurons than was observed in rats that did not step on the treadmill. There was a rostrocaudal and a dorsoventral gradient of FOS labeled neurons. The number of FOS+ neurons increased with the duration of treadmill stepping. Significant increases in FOS+ neurons were in the most medial parts of laminae IV, V, and VII. FOS+ motoneurons increased with treadmill stepping, particularly in large motoneurons (> or = 700 microm2).

CONCLUSION

These data suggest that FOS can be used to identify activity-dependent neuronal pathways in the spinal cord that are associated with treadmill stepping, specifically in lamina VII and in alpha motoneurons.

SPONSORSHIP

NIH NS16333, NS40917, and the Christopher Reeve Paralysis Foundation (CRPF VEC 2002).

Age-related disturbance of memory and CREB phosphorylation in CA1 area of hippocampus of rats

In the early process of long-term memory formation, cyclic AMP response element-binding protein (CREB), a transcription factor on which multiple signal transduction pathways converge, has been implicated. We examined whether the age difference in the performance of contextual fear conditioning (CFC) is associated with a change in activation of CREB in the hippocampus which is an important neural structure for long-term memory. The activation of CREB in the hippocampus in young (15 weeks old) and old (120 weeks old) male rats was determined immunohistochemically with an antibody that specifically recognizes the phosphorylated form of CREB (pCREB). Young rats exhibited better performance than old rats with respect to the freezing time in CFC. Phosphorylation of CREB as revealed by the ratio of the pCREB-immunoreactive cell number to the CREB-immunoreactive cell number was increased in the CA1 region, but not in other hippocampal regions following training for CFC. The close relationship between behavioral performance and CREB phosphorylation in the CA1 region suggests that hippocampal CREB is involved in age-related decline of learning and memory.

Molecular Mechanisms Underlying Emotional Learning and Memory in the Lateral Amygdala

Fear conditioning is a valuable behavioral paradigm for studying the neural basis of emotional learning and memory. The lateral nucleus of the amygdala (LA) is a crucial site of neural changes that occur during fear conditioning. Pharmacological manipulations of the LA, strategically timed with respect to training and testing, have shed light on the molecular events that mediate the acquisition of fear associations and the formation and maintenance of long-term memories of those associations. Similar mechanisms have been found to underlie long-term potentiation (LTP) in LA, an artificial means of inducing synaptic plasticity and a physiological model of learning and memory. Thus, LTP-like changes in synaptic plasticity may underlie fear conditioning. Given that the neural circuit underlying fear conditioning has been implicated in emotional disorders in humans, the molecular mechanisms of fear conditioning are potential targets for psychotherapeutic drug development.

Novel effects on memory observed following unilateral intracranial administration of okadaic acid, cyclosporin A, FK506 and [MeVal4]CyA

The involvement of protein phosphatases and peptidyl-prolyl cis/trans isomerases (PPIases) in memory formation in the chick has previously been investigated using a single-trial learning task. In these studies, inhibitory agents were administered bilaterally directly to a critical area of the chick brain. These studies are now extended to investigate whether similar effects are obtained if the drugs are administered unilaterally. All of the effects reported previously following bilateral administration of okadaic acid (OA), cyclosporin A (CyA), FK506 and [MeVal(4)]CyA can be attributed to their action in just one hemisphere. OA, at a concentration known to selectively inhibit PP2A in vitro (0.5 nM) results in permanent memory loss from 30-40 min post-training when injected in the left hemisphere, but has no effect when injected in the right hemisphere. A higher concentration of OA (100 nM), which inhibits both PP2A and PP1 in vitro, has a similar effect in the left hemisphere but causes a transient period of memory loss from 10-20 min post-training when injected in the right hemisphere. CyA (5 nM and 20 nM), which inhibits both PP2B and PPIase activity, causes permanent memory loss from 60 min post-training when injected into the left hemisphere, an effect also observed following administration of FK506 (20 nM), which also inhibits PP2B and PPIase activity, and [MeVal(4)]CyA (5 nM), which inhibits PPIase activity but not PP2B activity. Administration of CyA (20 nM) and FK506, but not [MeVal(4)]CyA, in the right hemisphere leads to a transient period of memory loss from 10-20 min post-training. These results confirm significant roles for phosphatases and PPIases in memory processing but challenge previous conclusions drawn on the basis of bilateral drug administration protocols.

Differential role of mitogen-activated protein kinase in three distinct phases of memory for sensitization in Aplysia

The mitogen-activated protein kinase (MAPK) pathway has been implicated recently in synaptic plasticity and memory. Here we used tail shock-induced sensitization of the tail-elicited siphon withdrawal reflex in Aplysia to examine the role of MAPK in three different phases of memory. We show that a specific pattern of serotonin (5-HT) application that produces intermediate-term and long-term synaptic facilitation (ITF and LTF, respectively) of the sensory-motor (SN-MN) synapses in Aplysia leads to sustained activation of extracellular signal-regulated kinase in the ventrocaudal cluster sensory neurons (SNs), which include the tail SNs. Furthermore, repeated tail shocks that induce intermediate-term and long-term memory (ITM and LTM, respectively) for sensitization also lead to sustained MAPK activation in the SNs. Given these results, we next examined the requirement of MAPK activity in (1) SN-MN synaptic facilitation and (2) memory for sensitization in Aplysia, by inhibiting MEK, the upstream kinase that phosphorylates and activates MAPK. In cellular experiments, we show that MAPK activity is required for ITF of tail SN-tail MN synapses, and, in parallel behavioral experiments, we show that ITM requires MAPK activity for its induction but not its expression. In contrast, short-term memory for sensitization does not require MAPK activity. Finally, 5-HT-induced LTF has been shown previously to require MAPK activity. Here we show that LTM for sensitization also requires MAPK activity. These results provide evidence that MAPK plays important roles specifically in long-lasting phases of synaptic plasticity and memory.

Mutation and activation of Galpha s similarly alters pre- and postsynaptic mechanisms modulating neurotransmission

Constitutive activation of Galphas in the Drosophila brain abolishes associative learning, a behavioral disruption far worse than that observed in any single cAMP metabolic mutant, suggesting that Galphas is essential for synaptic plasticity. The intent of this study was to examine the role of Galphas in regulating synaptic function by targeting constitutively active Galphas to either pre- or postsynaptic cells and by examining loss-of-function Galphas mutants (dgs) at the glutamatergic neuromuscular junction (NMJ) model synapse. Surprisingly, both loss of Galphas and activation of Galphas in either pre- or postsynaptic compartment similarly increased basal neurotransmission, decreased short-term plasticity (facilitation and augmentation), and abolished posttetanic potentiation. Elevated synaptic function was specific to an evoked neurotransmission pathway because both spontaneous synaptic vesicle fusion frequency and amplitude were unaltered in all mutants. In the postsynaptic cell, the glutamate receptor field was regulated by Galphas activity; based on immunocytochemical studies, GluRIIA receptor subunits were dramatically downregulated (>75% decrease) in both loss and constitutive active Galphas genotypes. In the presynaptic cell, the synaptic vesicle cycle was regulated by Galphas activity; based on FM1-43 dye imaging studies, evoked vesicle fusion rate was increased in both loss and constitutively active Galphas genotypes. An important conclusion of this study is that both increased and decreased Galphas activity very similarly alters pre- and postsynaptic mechanisms. A second important conclusion is that Galphas activity induces transynaptic signaling; targeted Galphas activation in the presynapse downregulates postsynaptic GluRIIA receptors, whereas targeted Galphas activation in the postsynapse enhances presynaptic vesicle cycling.

Insights into immediate-early gene function in hippocampal memory consolidation using antisense oligonucleotide and fluorescent imaging approaches

In the 14 years since it was discovered that specific genes could be dynamically regulated in the brain by neural activity, there has been a substantial research focus attempting to understand the role immediate-early genes (IEGs) play in various brain functions. This article examines the involvement of IEGs in hippocampal synaptic plasticity and in memory consolidation processes performed by the hippocampus. Studies employing conventional IEG detection methodologies and a novel gene-imaging approach that provides temporal and cellular resolution (cellular compartment analysis of emporal activity by fluorescence in situ hybridization or catFISH) provide evidence supporting the assertion that IEG expression reflects the integration of information processed by hippocampal neurons. However, IEG expression is not merely correlated with neural activity, but also plays a pivotal role in stabilizing recent changes in synaptic efficacy. As such, localized disruption of IEGs Arc or c-fos by intrahippocampal administration of antisense oligonucleotides or germline disruption of the IEGs c-fos, tissue plasminogen activator, or zif268 impairs consolidation of long-term memory formation, without affecting learning or short-term memory. Further investigation into the expression and function of IEGs using catFISH and antisense approaches will likely increase understanding of the molecular and cellular bases of information processing involving the hippocampus.

Long-term depression in the adult hippocampus in vivo involves activation of extracellular signal-regulated kinase and phosphorylation of Elk-1

Protein kinase cascades likely play a critical role in the signaling events that underlie synaptic plasticity and memory. The extracellular signal-regulated kinase (ERK) cascade is suited well for such a role because its targets include regulators of gene expression. Here we report that the ERK cascade is recruited during long-term depression (LTD) of synaptic strength in area CA1 of the adult hippocampus in vivo and selectively impacts on phosphorylation of the nuclear transcription factor Elk-1. Using a combination of in vivo electrophysiology, biochemistry, pharmacology, and immunohistochemistry, we found the following: (1) ERK phosphorylation, including phosphorylation of nuclear ERK, and ERK phosphotransferase activity are increased markedly, albeit transiently, after the induction of NMDA receptor-dependent LTD at the commissural input to area CA1 pyramidal cells in the hippocampus of anesthetized adult rats; (2) LTD-inducing paired-pulse stimulation fails to produce lasting LTD in the presence of the ERK kinase inhibitor SL327, which suggests that ERK activation is necessary for the persistence of LTD; and (3) ERK activation during LTD results in increased phosphorylation of Elk-1 but not of the transcription factor cAMP response element-binding protein. Our findings indicate that the ERK cascade transduces signals from the synapse to the nucleus during LTD in hippocampal area CA1 in vivo, as it does during long-term potentiation in area CA1, but that the pattern of coupling of the ERK cascade to transcriptional regulators differs between the two forms of synaptic plasticity.

Basolateral amygdala lesions block the memory-enhancing effect of 8-Br-cAMP infused into the entorhinal cortex of rats after training

There is extensive evidence suggesting that the basolateral nucleus of the amygdala plays a critical role in modulating memory consolidation processes in other brain regions. The present experiments examined interactions between the basolateral amygdala and the entorhinal cortex in modulating memory consolidation for inhibitory avoidance training. Several studies have reported that activation of the second messenger system adenosine 3',5'-cyclic monophosphate (cAMP) in several brain regions enhances memory and induces long-term plasticity. In the present experiments, a unilateral infusion of the cAMP analogue, 8-Br-cAMP (0.25 or 1.25 microg in 0.5 microL), administered into the entorhinal cortex of male Sprague-Dawley rats immediately after training, enhanced 48-h retention. An N-methyl-d-aspartate-induced lesion of the ipsilateral basolateral amygdala did not impair retention, but blocked the memory-enhancing effect of 8-Br-cAMP (infused into the entorhinal cortex) post-training. A lesion of the contralateral basolateral amygdala did not block the 8-Br-cAMP-induced retention enhancement. These findings indicate that an intact basolateral amygdala is essential for modulation of memory consolidation involving the entorhinal cortex, and are consistent with evidence that the basolateral amygdala regulates memory consolidation mediated by other brain regions.

Memory consolidation of Pavlovian fear conditioning: a cellular and molecular perspective

Pavlovian fear conditioning has emerged as a leading behavioral paradigm for studying the neurobiological basis of learning and memory. Although considerable progress has been made in understanding the neural substrates of fear conditioning at the systems level, until recently little has been learned about the underlying cellular and molecular mechanisms. The success of systems-level work aimed at defining the neuroanatomical pathways underlying fear conditioning, combined with the knowledge accumulated by studies of long-term potentiation (LTP), has recently given way to new insights into the cellular and molecular mechanisms that underlie acquisition and consolidation of fear memories. Collectively, these findings suggest that fear memory consolidation in the amygdala shares essential biochemical features with LTP, and hold promise for understanding the relationship between memory consolidation and synaptic plasticity in the mammalian brain.

Modulation of protein kinase A activation by fibronectin matrix proteins at developing neuromuscular synapses in Xenopus laevis cell cultures

Extracellular matrix proteins, such as fibronectin, laminin, and collagen, have been implicated in a wide variety of cellular properties, which include cell adhesion, migration, differentiation, and proliferation. In this study, we investigated the modulation of protein kinase A (PKA) activity by matrix proteins at developing motoneurons. The cultures of spinal neurons and myotomal cells were prepared from 1-day-old Xenopus laevis embryos. Spontaneous synaptic currents (SSC) were recorded from innervated myocytes of natural synapses by whole-cell voltage-clamped recordings (V(h) = -60 to approximately -65 mV). Bath application of agents, which directly or indirectly activate PKA, such as forskolin (20 microM), dibutyryl cAMP (DBcAMP) (1 mM), isoproterenol (10 microM), or albuterol (10 microM), significantly increased SSC frequency in cultures grown on fibronectin (FN)-coated substratum, but not on laminin- or collagen-coated glasses. The evoked synaptic currents increased in response to forskolin in neurons grown on FN substratum. Triflavin, an Arg-Gly-Asp-dependent disintegrin, inhibited potentiating action of isoproterenol in neurons grown on FN substratum, suggesting that integrin is involved in the potentiation of the PKA pathway in the regulation of acetylcholine (ACh) release. There is collaboration of neurotrophic factors and the FN matrix in regulating synaptic transmission in response to DBcAMP. Chronic treatment with neurotrophic factors, such as ciliary neurotrophic factor (150 ng/ml), glial cell line-derived neurotrophic factor (30 ng/ml), or neurotrophin-3 (50 ng/ml), enhanced the SSC-increasing action of DBcAMP in neurons grown on FN-coated glasses. These results suggest that the FN matrix potentiates synaptic transmission in response to PKA activation. Neurotrophic factors may collaborate with FN to regulate spontaneous ACh secretion at developing motoneurons, which may play an important role in the maturation of embryonic neuromuscular synapses.

The amygdala modulates memory consolidation of fear-motivated inhibitory avoidance learning but not classical fear conditioning

Although the lateral and basal nuclei of the amygdala are believed to be essential for the acquisition of Pavlovian fear conditioning, studies using post-training manipulations of the amygdala in the inhibitory avoidance learning paradigm have recently called this view into question. We used the GABA(A) agonist muscimol to functionally inactivate these nuclei immediately after single-trial Pavlovian fear conditioning or single-trial inhibitory avoidance learning. Immediate post-training infusions of muscimol had no effect on Pavlovian conditioning but produced a dose-dependent effect on inhibitory avoidance. However, pre-training infusions dose-dependently disrupted Pavlovian conditioning. These findings indicate that the amygdala plays an essential role in the acquisition of Pavlovian fear conditioning and contributes to the modulation of memory consolidation of inhibitory avoidance but not of Pavlovian fear conditioning.

Theoretical aspects of the neurobiological integration of memory

Whenever a peripheral structure like the visual system captures information, the input signal reverberates in circuits of neurons, which send it thereafter towards: (a) the motor system, triggering a specific response, evoked by a short-term memory mechanism; and (b) the hippocampus, to produce long-term potentiation or depression. Two different processes regulate short-term memory: (1) Homosynaptic depression that inhibits neurotransmitter release by means of a decrease in Ca++ inflow, and an increase in calmodulin affinity for synaptic vesicles; and (2) Heterosynaptic facilitation that triggers neurotransmitter release, whenever serotonin activates a proteinkinase A. Besides carrying out a brief review on the matter, we support two different physiological explanations with regard to: (a) ion exchange process and the interstitial pH during habituation; and (b) the possibility of a sensitive presynaptic neuron interaction within the habituated reverberant circuit, to trigger dishabituation. We also propose the term 'time-mediated stimulatory action dependent' to name those serotonin receptors that may lead to a rapid or a delayed postsynaptic onset responses.

A mitogen-activated protein kinase cascade in the CA1/CA2 subfield of the dorsal hippocampus is essential for long-term spatial memory

Behavioral, biophysical, and pharmacological studies have implicated the hippocampus in the formation and storage of spatial memory. However, the molecular mechanisms underlying long-term spatial memory are poorly understood. In this study, we show that mitogen-activated protein kinase (MAPK, also called ERK) is activated in the dorsal, but not the ventral, hippocampus of rats after training in a spatial memory task, the Morris water maze. The activation was expressed as enhanced phosphorylation of MAPK in the pyramidal neurons of the CA1/CA2 subfield. In contrast, no increase in the percentage of phospho-MAPK-positive cells was detected in either the CA3 subfield or the dentate gyrus. The enhanced phosphorylation was observed only after multiple training trials but not after a single trial or after multiple trials in which the location of the target platform was randomly changed between each trial. Inhibition of the MAPK/ERK cascade in dorsal hippocampi did not impair acquisition, but blocked the formation of long-term spatial memory. In contrast, intrahippocampal infusion of SB203580, a specific inhibitor of the stress-activated MAPK (p38 MAPK), did not interfere with memory storage. These results demonstrate a MAPK-mediated cellular event in the CA1/CA2 subfields of the dorsal hippocampus that is critical for long-term spatial memory.

Memory Consolidation for Contextual and Auditory Fear Conditioning Is Dependent on Protein Synthesis, PKA, and MAP Kinase

Fear conditioning has received extensive experimental attention. However, little is known about the molecular mechanisms that underlie fear memory consolidation. Previous studies have shown that long-term potentiation (LTP) exists in pathways known to be relevant to fear conditioning and that fear conditioning modifies neural processing in these pathways in a manner similar to LTP induction. The present experiments examined whether inhibition of protein synthesis, PKA, and MAP kinase activity, treatments that block LTP, also interfere with the consolidation of fear conditioning. Rats were injected intraventricularly with Anisomycin (100 or 300 μg), Rp-cAMPS (90 or 180 μg), or PD098059 (1 or 3 μg) prior to conditioning and assessed for retention of contextual and auditory fear memory both within an hour and 24 hr later. Results indicated that injection of these compounds selectively interfered with long-term memory for contextual and auditory fear, while leaving short-term memory intact. Additional control groups indicated that this effect was likely due to impaired memory consolidation rather than to nonspecific effects of the drugs on fear expression. Results suggest that fear conditioning and LTP may share common molecular mechanisms.

Differential phosphorylation of syntaxin and synaptosome-associated protein of 25 kDa (SNAP-25) isoforms

The synaptic plasma membrane proteins syntaxin and synaptosome-associated protein of 25 kDa (SNAP-25) are central participants in synaptic vesicle trafficking and neurotransmitter release. Together with the synaptic vesicle protein synaptobrevin/vesicle-associated membrane protein (VAMP), they serve as receptors for the general membrane trafficking factors N-ethylmaleimide-sensitive factor (NSF) and soluble NSF attachment protein (alpha-SNAP). Consequently, syntaxin, SNAP-25, and VAMP (and their isoforms in other membrane trafficking pathways) have been termed SNAP receptors (SNAREs). Because protein phosphorylation is a common and important mechanism for regulating a variety of cellular processes, including synaptic transmission, we have investigated the ability of syntaxin and SNAP-25 isoforms to serve as substrates for a variety of serine/threonine protein kinases. Syntaxins 1 A and 4 were phosphorylated by casein kinase II, whereas syntaxin 3 and SNAP-25 were phosphorylated by Ca2+- and calmodulin-dependent protein kinase II and cyclic AMP-dependent protein kinase, respectively. The biochemical consequences of SNARE protein phosphorylation included a reduced interaction between SNAP-25 and phosphorylated syntaxin 4 and an enhanced interaction between phosphorylated syntaxin 1A and the synaptic vesicle protein synaptotagmin I, a potential Ca2+ sensor in triggering synaptic vesicle exocytosis. No other effects on the formation of SNARE complexes (comprised of syntaxin, SNAP-25, and VAMP) or interactions involving n-Sec1 or alpha-SNAP were observed. These findings suggest that although phosphorylation does not directly regulate the assembly of the synaptic SNARE complex, it may serve to modulate SNARE complex function through other proteins, including synaptotagmin I.

Recovery of memory in chicks after disruption during learning: the reversibility of amnesia induced by protein synthesis inhibitors

Protein synthesis inhibitors given during learning are known to disrupt memory in various animal species in several models of learning. However, there are suggestions that amnesia induced by protein synthesis inhibitors is not permanent--memory can be recovered by a reminder procedure, i.e., by presenting the animal with one of the components of the external environment which was part of the learning situation. The aim of the present work was to determine the existence of the reminder phenomenon in a well-studied model of single-session training to passive avoidance in chicks. Cycloheximide and anisomycin were used to induce amnesia. Reminder was performed using the aversive taste of methylanthranilate 24 h after training, and testing was conducted 48 h after training. The results obtained provide evidence that memory disrupted by protein synthesis inhibitors in chicks can be recovered by the reminder procedure.

Detection of gene expression in single neurons by patch-clamp and single-cell reverse transcriptase polymerase chain reaction

Detection and quantitation of gene expression in single cells is especially important in the central nervous system where, at the cellular level, the synapse can be considered the single functional unit. For example, the consolidation of long-term memories may be mediated by persistent changes in the strength of synaptic transmission at individual synapses. In order to investigate the requirement for de novo RNA synthesis during long-term potentiation in individual neurons, we have combined single-cell electrophysiology with single-cell gene-expression methodology. Described are methods combining whole-cell patch-clamp and single-cell RT-PCR for the detection of a single mRNA species for nitric oxide synthase, or, through a multiplex strategy, for the simultaneous detection of several mRNAs including heme oxygenase 2, protein phosphatase inhibitor 1 protein, and several isoforms of the calcium/calmodulin dependent protein kinase II.

Protein synthesis-dependent memory and neuronal enhancement in Hermissenda are contingent on parameters of training and retention

Following contiguous pairings of light and rotation, light alone elicits a conditioned contraction of Hermissenda's foot, indicative of an associative memory. After a 5-min retention interval, this conditioned response was evident following two or nine (but not one) conditioning trials but persisted for 90 min only after nine trials. In vivo incubation of animals in the protein synthesis inhibitor anisomycin (ANI; 1 microM) did not affect the conditioned response at the 5-min retention interval but significantly attenuated conditioned responding at the 90-min interval even following nine training trials. Deacetylanisomycin (DANI; 1 microM; an inactive form of anisomycin) had no effect on either 5- or 90-min retention. In a companion procedure, groups of isolated nervous systems were exposed to comparable light and rotation pairings, and the B photoreceptors (considered a site of storage for the associative memory) underwent electrophysiological analysis. An increase in neuronal excitability (indexed by depolarizing voltage responses to injected current) in the B photoreceptors paralleled the expression of conditioned responding in intact animals, that is, two training trials produced a short-term increase in excitability that dissipated within 45 min, whereas nine trials produced a persistent (at least 90-min) increase in excitability. In a fmal experiment, isolated nervous systems were exposed to nine training trials, and ANI or DANI was either present in the bathing medium before and during training or was introduced 5 min after training. Following training in ANI, a short-term (5- to 45-min) but not persistent (90-min) increase in excitability in the B photoreceptors was observed. ANI had no effect on either the short-term or persistent increase in excitability if the drug was applied 5 min after the last (ninth) training trial, and DANI had no effect on training-induced increases in excitability at any retention intervals. These results suggest that short-term retention in Hermissenda is protein synthesis independent but that new protein synthesis initiated during or shortly after the training event is necessary for even 90-min retention. Moreover, these results indicate that under some conditions, a critical threshold of training must be exceeded to initiate protein synthesis-dependent retention.