Specificity of molecular changes in neurons involved in memory storage

Neurogranin Regulates Metaplasticity

Long-term potentiation (LTP) and long-term depression (LTD) are two major forms of synaptic plasticity that are widely accepted as cellular mechanisms involved in learning and memory. Metaplasticity is a process whereby modifications in synaptic processes shift the threshold for subsequent plasticity. While metaplasticity has been functionally observed, its molecular basis is not well understood. Here, we report that neurogranin (Ng) regulates metaplasticity by shifting the threshold toward potentiation, i.e., increasing Ng in hippocampal neurons lowers the threshold for LTP and augments the threshold for LTD. We also show that Ng does not change the ultrastructural localization of calmodulin (CaM)-dependent protein Kinase II (CaMKII) or calcineurin, critical enzymes for the induction of LTP and LTD, respectively. Interestingly, while CaMKII concentrates close to the plasma membrane, calcineurin concentrates away from the plasma membrane. These data, along with the previous observation showing Ng targets CaM closer to the plasma membrane, suggesting that shifting the localization of CaM within the dendritic spines and closer to the plasma membrane, where there is more CaMKII, may be favoring the activation of CaMKII vs. that of calcineurin. Thus, the regulation of CaM localization/targeting within dendritic spines by Ng may provide a mechanistic basis for the regulation of metaplasticity.

Adult neurogenesis and neurodegenerative diseases: A systems biology perspective

New neurons are generated throughout adulthood in two regions of the brain, the olfactory bulb and dentate gyrus of the hippocampus, and are incorporated into the hippocampal network circuitry; disruption of this process has been postulated to contribute to neurodegenerative diseases including Alzheimer's disease and Parkinson's disease. Known modulators of adult neurogenesis include signal transduction pathways, the vascular and immune systems, metabolic factors, and epigenetic regulation. Multiple intrinsic and extrinsic factors such as neurotrophic factors, transcription factors, and cell cycle regulators control neural stem cell proliferation, maintenance in the adult neurogenic niche, and differentiation into mature neurons; these factors act in networks of signaling molecules that influence each other during construction and maintenance of neural circuits, and in turn contribute to learning and memory. The immune system and vascular system are necessary for neuronal formation and neural stem cell fate determination. Inflammatory cytokines regulate adult neurogenesis in response to immune system activation, whereas the vasculature regulates the neural stem cell niche. Vasculature, immune/support cell populations (microglia/astrocytes), adhesion molecules, growth factors, and the extracellular matrix also provide a homing environment for neural stem cells. Epigenetic changes during hippocampal neurogenesis also impact memory and learning. Some genetic variations in neurogenesis related genes may play important roles in the alteration of neural stem cells differentiation into new born neurons during adult neurogenesis, with important therapeutic implications. In this review, we discuss mechanisms of and interactions between these modulators of adult neurogenesis, as well as implications for neurodegenerative disease and current therapeutic research. © 2016 Wiley Periodicals, Inc.

Neurogranin regulates CaM dynamics at dendritic spines

Calmodulin (CaM) plays a key role in synaptic function and plasticity due to its ability to mediate Ca²⁺ signaling. Therefore, it is essential to understand the dynamics of CaM at dendritic spines. In this study we have explored CaM dynamics using live-cell confocal microscopy and fluorescence recovery after photobleaching (FRAP) to study CaM diffusion. We find that only a small fraction of CaM in dendritic spines is immobile. Furthermore, the diffusion rate of CaM was regulated by neurogranin (Ng), a CaM-binding protein enriched at dendritic spines. Interestingly, Ng did not influence the immobile fraction of CaM at recovery plateau. We have previously shown that Ng enhances synaptic strength in a CaM-dependent manner. Taken together, these data indicate that Ng-mediated enhancement of synaptic strength is due to its ability to target, rather than sequester, CaM within dendritic spines.

X-ray, spectroscopic and normal-mode dynamics of calexcitin: structure-function studies of a neuronal calcium-signalling protein

The protein calexcitin was originally identified in molluscan photoreceptor neurons as a 20 kDa molecule which was up-regulated and phosphorylated following a Pavlovian conditioning protocol. Subsequent studies showed that calexcitin regulates the voltage-dependent potassium channel and the calcium-dependent potassium channel as well as causing the release of calcium ions from the endoplasmic reticulum (ER) by binding to the ryanodine receptor. A crystal structure of calexcitin from the squid Loligo pealei showed that the fold is similar to that of another signalling protein, calmodulin, the N- and C-terminal domains of which are known to separate upon calcium binding, allowing interactions with the target protein. Phosphorylation of calexcitin causes it to translocate to the cell membrane, where its effects on membrane excitability are exerted and, accordingly, L. pealei calexcitin contains two protein kinase C phosphorylation sites (Thr61 and Thr188). Thr-to-Asp mutations which mimic phosphorylation of the protein were introduced and crystal structures of the corresponding single and double mutants were determined, which suggest that the C-terminal phosphorylation site (Thr188) exerts the greatest effects on the protein structure. Extensive NMR studies were also conducted, which demonstrate that the wild-type protein predominantly adopts a more open conformation in solution than the crystallographic studies have indicated and, accordingly, normal-mode dynamic simulations suggest that it has considerably greater capacity for flexible motion than the X-ray studies had suggested. Like calmodulin, calexcitin consists of four EF-hand motifs, although only the first three EF-hands of calexcitin are involved in binding calcium ions; the C-terminal EF-hand lacks the appropriate amino acids. Hence, calexcitin possesses two functional EF-hands in close proximity in its N-terminal domain and one functional calcium site in its C-terminal domain. There is evidence that the protein has two markedly different affinities for calcium ions, the weaker of which is most likely to be associated with binding of calcium ions to the protein during neuronal excitation. In the current study, site-directed mutagenesis has been used to abolish each of the three calcium-binding sites of calexcitin, and these experiments suggest that it is the single calcium-binding site in the C-terminal domain of the protein which is likely to have a sensory role in the neuron.

A role for histone deacetylases in the cellular and behavioral mechanisms underlying learning and memory

Histone deacetylases (HDACs) are a family of chromatin remodeling enzymes that restrict access of transcription factors to the DNA, thereby repressing gene expression. In contrast, histone acetyltransferases (HATs) relax the chromatin structure allowing for an active chromatin state and promoting gene transcription. Accumulating data have demonstrated a crucial function for histone acetylation and histone deacetylation in regulating the cellular and behavioral mechanisms underlying synaptic plasticity and learning and memory. In trying to delineate the roles of individual HDACs, genetic tools have been used to manipulate HDAC expression in rodents, uncovering distinct contributions of individual HDACs in regulating the processes of memory formation. Moreover, recent findings have suggested an important role for HDAC inhibitors in enhancing learning and memory processes as well as ameliorating symptoms related to neurodegenerative diseases. In this review, we focus on the role of HDACs in learning and memory, as well as significant data emerging from the field in support of HDAC inhibitors as potential therapeutic targets for the treatment of cognitive disorders.

Mucus trail tracking in a predatory snail: olfactory processing retooled to serve a novel sensory modality

INTRODUCTION

The rosy wolfsnail (Euglandina rosea), a predatory land snail, finds prey snails and potential mates by following their mucus trails. Euglandina have evolved unique, mobile lip extensions that detect mucus and aid in following trails. Currently, little is known of the neural substrates of the trail-following behavior.

METHODS

To investigate the neural correlates of trail following we used tract-tracing experiments in which nerves were backfilled with either nickel-lysine or Lucifer yellow, extracellular recording of spiking neurons in snail procerebra using a multielectrode array, and behavioral assays of trail following and movement toward the source of a conditioned odor.

RESULTS

The tract-tracing experiments demonstrate that in Euglandina, the nerves carrying mucus signals innervate the same region of the central ganglia as the olfactory nerves, while the electrophysiology studies show that mucus stimulation of the sensory epithelium on the lip extensions alters the frequency and pattern of neural activity in the procerebrum in a manner similar to odor stimulation of the olfactory epithelium on the optic tentacles of another land snail species, Cantareus aspersa (previously known as Helix aspersa). While Euglandina learn to follow trails of novel chemicals that they contact with their lip extensions in one to three trials, these snails proved remarkably resistant to associative learning in the olfactory modality. Even after seven to nine pairings of odorant molecules with food, they showed no orientation toward the conditioned odor. This is in marked contrast to Cantareus snails, which reliably oriented toward conditioned odors after two to three trials.

CONCLUSIONS

The apparent inability of Euglandina to learn to associate food with odors and use odor cues to drive behavior suggests that the capability for sophisticated neural processing of nonvolatile mucus cues detected by the lip extensions has evolved at the expense of processing of odorant molecules detected by the olfactory system.

Chemokines and the hippocampus: a new perspective on hippocampal plasticity and vulnerability

The hippocampus is critical for several aspects of learning and memory and is unique among other cortical regions in structure, function and the potential for plasticity. This remarkable region recapitulates development throughout the lifespan with enduring neurogenesis and well-characterized plasticity. The structure and traits of the hippocampus that distinguish it from other brain regions, however, may be the same reasons that this important brain region is particularly vulnerable to insult and injury. The immune system within the brain responds to insult and injury, and the hippocampus and the immune system are extensively interconnected. Immune signaling molecules, cytokines and chemokines (chemotactic cytokines), are well known for their functions during insult or injury. They are also increasingly implicated in normal hippocampal neurogenesis (e.g., CXCR4 on newborn neurons), cellular plasticity (e.g., interleukin-6 in LTP maintenance), and learning and memory (e.g., interleukin-1β in fear conditioning). We provide evidence from the small but growing literature that neuroimmune interactions and immune signaling molecules, especially chemokines, may be a primary underlying mechanism for the coexistence of plasticity and vulnerability within the hippocampus. We also highlight the evidence that the hippocampus exhibits a remarkable resilience in response to diverse environmental events (e.g., enrichment, exercise), which all may converge onto common neuroimmune mechanisms.

Neurogranin targets calmodulin and lowers the threshold for the induction of long-term potentiation

Calcium entry and the subsequent activation of CaMKII trigger synaptic plasticity in many brain regions. The induction of long-term potentiation (LTP) in the CA1 region of the hippocampus requires a relatively high amount of calcium-calmodulin. This requirement is usually explained, based on in vitro and theoretical studies, by the low affinity of CaMKII for calmodulin. An untested hypothesis, however, is that calmodulin is not randomly distributed within the spine and its targeting within the spine regulates LTP. We have previously shown that overexpression of neurogranin enhances synaptic strength in a calmodulin-dependent manner. Here, using post-embedding immunogold labeling, we show that calmodulin is not randomly distributed, but spatially organized in the spine. Moreover, neurogranin regulates calmodulin distribution such that its overexpression concentrates calmodulin closer to the plasma membrane, where a high level of CaMKII immunogold labeling is also found. Interestingly, the targeting of calmodulin by neurogranin results in lowering the threshold for LTP induction. These findings highlight the significance of calmodulin targeting within the spine in synaptic plasticity.

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.

Neurogranin and synaptic plasticity balance

Learning-related modifications of synaptic transmission at CA1 hippocampal excitatory synapses are activity- and NMDA receptor (NMDAR)-dependent. While a postsynaptic increase in Ca(2+) is absolutely required for synaptic plasticity induction, the molecular mechanisms underlying the transduction of synaptic signals to postsynaptic changes are not clearly understood. In our recent study, we found that the postsynaptic calmodulin (CaM)-binding protein neurogranin (Ng) enhances synaptic strength in an activity- and NMDAR-dependent manner. Furthermore we have shown that Ng is not only required for the induction of long-term potentiation (LTP), but its mediated synaptic potentiation also mimics and occludes LTP. Our results demonstrate that Ng plays an important role in the regulation of hippocampal synaptic plasticity and synaptic function. Here, we summarize our findings and further discuss their possible implications in aging-related synaptic plasticity deficits.

Neurogranin phosphorylation fine-tunes long-term potentiation

Learning-related potentiation of synaptic strength at Cornu ammonis subfield 1 (CA1) hippocampal excitatory synapses is dependent on neuronal activity and the activation of glutamate receptors. However, molecular mechanisms that regulate and fine-tune the expression of long-term potentiation (LTP) are not well understood. Recently it has been indicated that neurogranin (Ng), a neuron-specific, postsynaptic protein that is phosphorylated by protein kinase C, potentiates synaptic transmission in an LTP-like manner. Here, we report that a Ng mutant that is unable to be phosphorylated cannot potentiate synaptic transmission in rat CA1 hippocampal neurons and results in a submaximal expression of LTP. Our results provide the first evidence that the phosphorylation of Ng can regulate LTP expression.

The function of GluR1 and GluR2 in cerebellar and hippocampal LTP and LTD is regulated by interplay of phosphorylation and O-GlcNAc modification

Long-term potentiation (LTP) and long-term depression (LTD) are the current models of synaptic plasticity and widely believed to explain how different kinds of memory are stored in different brain regions. Induction of LTP and LTD in different regions of brain undoubtedly involve trafficking of AMPA receptor to and from synapses. Hippocampal LTP involves phosphorylation of GluR1 subunit of AMPA receptor and its delivery to synapse whereas; LTD is the result of dephosphorylation and endocytosis of GluR1 containing AMPA receptor. Conversely the cerebellar LTD is maintained by the phosphorylation of GluR2 which promotes receptor endocytosis while dephosphorylation of GluR2 triggers receptor expression at the cell surface and results in LTP. The interplay of phosphorylation and O-GlcNAc modification is known as functional switch in many neuronal proteins. In this study it is hypothesized that a same phenomenon underlies as LTD and LTP switching, by predicting the potential of different Ser/Thr residues for phosphorylation, O-GlcNAc modification and their possible interplay. We suggest the involvement of O-GlcNAc modification of dephosphorylated GluR1 in maintaining the hippocampal LTD and that of dephosphorylated GluR2 in cerebral LTP.

Proteomic analysis of post-translational modifications in conditioned Hermissenda

Post-translational modifications of proteins are a major determinant of biological function. Phosphorylation of proteins involved in signal transduction contributes to the induction and maintenance of several examples of cellular and synaptic plasticity. In this study we have identified phosphoproteins regulated by Pavlovian conditioning in lysates of Hermissenda nervous systems using two-dimensional electrophoresis (2DE) in conjunction with (32)P labeling, fluorescence based phosphoprotein in-gel staining, and mass spectrometry. Modification of protein phosphorylation regulated by conditioning was first assessed by densitometric analysis of (32)P labeled proteins resolved by 2DE from lysates of conditioned and pseudorandom control nervous systems. An independent assessment of phosphorylation regulated by conditioning was obtained from an examination of 2D gels stained with Pro-Q Diamond phosphoprotein dye. Mass spectrometric analysis of protein digests from phosphoprotein stained analytical gels or Coomassie Blue stained preparative gels provided for the identification of phosphoproteins that exhibited statistically significant increased phosphorylation in conditioned groups as compared to pseudorandom controls. A previously identified cytoskeletal related protein, Csp24 (24 kDa conditioned stimulus pathway phosphoprotein), involved in intermediate-term memory exhibited significantly increased phosphorylation detected 24 h post-conditioning. Our results show that proteins involved in diverse cellular functions such as transcriptional regulation, cell signaling, cytoskeletal regulation, metabolic activity, and protein degradation contribute to long-term post-translational modifications associated with Pavlovian conditioning.

Neurogranin enhances synaptic strength through its interaction with calmodulin

Learning-correlated plasticity at CA1 hippocampal excitatory synapses is dependent on neuronal activity and NMDA receptor (NMDAR) activation. However, the molecular mechanisms that transduce plasticity stimuli to postsynaptic potentiation are poorly understood. Here, we report that neurogranin (Ng), a neuron-specific and postsynaptic protein, enhances postsynaptic sensitivity and increases synaptic strength in an activity- and NMDAR-dependent manner. In addition, Ng-mediated potentiation of synaptic transmission mimics and occludes long-term potentiation (LTP). Expression of Ng mutants that lack the ability to bind to, or dissociate from, calmodulin (CaM) fails to potentiate synaptic transmission, strongly suggesting that regulated Ng-CaM binding is necessary for Ng-mediated potentiation. Moreover, knocking-down Ng blocked LTP induction. Thus, Ng-CaM interaction can provide a mechanistic link between induction and expression of postsynaptic potentiation.

Recurrent affective disorder: Roots in developmental neurobiology and illness progression based on changes in gene expression

Electrophysiological kindling and behavioral sensitization to psychomotor stimulants and stress provide paradigms for understanding how repeated acute events can leave neurobiological residues in gene expression, accounting for the observed long-lasting alterations in behavioral responsivity. Kindling helps conceptualize how repeated electrical stimulation of the brain can progressively evoke increased behavioral and convulsive responsivity, leading to spontaneous seizures in the absence of exogenous stimulation following sufficient stimulations. As kindling unfolds, a complex spatiotemporal cascade of events occurs and includes the induction of immediate early genes (e.g.,c-fos) and late effector genes (including peptides and growth factors) possibly associated with the observed changes in brain microstructure (e.g., synapse formation, axonal and dendritic sprouting, apoptosis). Behavioral sensitization to psychomotor stimulants and stress has also been shown to induce related but different cascades of effects on immediate early and late effector gene expression. These may be associated with the observed long-lasting alterations in behavioral responsivity based on prior experience. If these types of alterations are put into a developmental context, this would provide a paradigm for understanding how early life events could exert profound and behaviorally relevant biochemical and microstructural effects on the central nervous system of the developing organism. The conceptual overview offered by the sensitization and kindling models suggests that environmentally triggered neurobiological processes do not form a single or static residue but, instead, engage processes related to developmental neurobiology and learning and memory and whose substrate is constantly evolving over an organism's lifetime.

[Acquiring new information in a neuronal network: from Hebb's concept to homeostatic plasticity]

Synaptic plasticity is the cellular mechanism underlying the phenomena of learning and memory. Much of the research on synaptic plasticity is based on the postulate of Hebb (1949) who proposed that, when a neuron repeatedly takes part in the activation of another neuron, the efficacy of the connections between these neurons is increased. Plasticity has been extensively studied, and often demonstrated through the processes of LTP (Long Term Potentiation) and LTD (Long Term Depression), which represent an increase and a decrease of the efficacy of long-term synaptic transmission. This review summarizes current knowledge concerning the cellular mechanisms of LTP and LTD, whether at the level of excitatory synapses, which have been the most studied, or at the level of inhibitory synapses. However, if we consider neuronal networks rather than the individual synapses, the consequences of synaptic plasticity need to be considered on a large scale to determine if the activity of networks are changed or not. Homeostatic plasticity takes into account the mechanisms which control the efficacy of synaptic transmission for all the synaptic inputs of a neuron. Consequently, this new concept deals with the coordinated activity of excitatory and inhibitory networks afferent to a neuron which maintain a controlled level of excitability during the acquisition of new information related to the potentiation or to the depression of synaptic efficacy. We propose that the protocols of stimulation used to induce plasticity at the synaptic level set up a "homeostatic potentiation" or a "homeostatic depression" of excitation and inhibition at the level of the neuronal networks. The coordination between excitatory and inhibitory circuits allows the neuronal networks to preserve a level of stable activity, thus avoiding episodes of hyper- or hypo-activity during the learning and memory phases.

Coregulation of light neurofilament mRNA by poly(A)-binding protein and aldolase C: Implications for neurodegeneration

The multifunctional proteins aldolase C and poly (A)-binding protein (PABP) undergo competitive interactions in cells coexpressing aldolase C and NF-L. A specific in vivo interaction between aldolase C and NF-L mRNA had been localized to a 68 nt segment of the transcript spanning the translation termination signal. It is shown here that the poly (A)-binding protein (PABP) binds the body of the NF-L transcript and increases its levels of expression when an excess of PABP is transiently provided in trans. Immunoprecipitation of PABP-associated ribonucleoprotein complexes of human spinal cord pulls down the dimeric form of aldolase C suggesting that their co-regulation of NF-L expression could be linked to the oligomerization status of aldolase C. An ex vivo model of mRNA decay has assessed mechanisms whereby aldolase C and PABP control NF-L expression. This model shows that aldolase C is a zinc-activated ribonuclease that cleaves the transcript at sites closed to the end-terminal structures. Immunological and biochemical depletion of endogenous PABP increases the instability of the transcript suggesting that PABP shields the NF-L mRNA from aldolase attack. An in vitro model shows that a mutant NF-L 68, in which the 45 nt of proximal 3'-UTR is replaced with unrelated sequence, is not degraded by aldolase C. Taken together, the findings might have important consequences for understanding causal mechanisms underlying neurodegeneration.

Bryostatin enhancement of memory in Hermissenda

Bryostatin, a potent agonist of protein kinase C (PKC), when administered to Hermissenda was found to affect acquisition of an associative learning paradigm. Low bryostatin concentrations (0.1 to 0.5 ng/ml) enhanced memory acquisition, while concentrations higher than 1.0 ng/ml down-regulated the pathway and no recall of the associative training was exhibited. The extent of enhancement depended upon the conditioning regime used and the memory stage normally fostered by that regime. The effects of two training events (TEs) with paired conditioned and unconditioned stimuli, which standardly evoked only short-term memory (STM) lasting 7 min, were--when bryostatin was added concurrently--enhanced to a long-term memory (LTM) that lasted about 20 h. The effects of both 4- and 6-paired TEs (which by themselves did not generate LTM), were also enhanced by bryostatin to induce a consolidated memory (CM) that lasted at least 5 days. The standard positive 9-TE regime typically produced a CM lasting at least 6 days. Low concentrations of bryostatin (<0.5 ng/ml) elicited no demonstrable enhancement of CM from 9-TEs. However, animals exposed to bryostatin concentrations higher than 1.0 ng/ml exhibited no behavioral learning. Sharp-electrode intracellular recordings of type-B photoreceptors in the eyes from animals conditioned in vivo with bryostatin revealed changes in input resistance and an enhanced long-lasting depolarization (LLD) in response to light. Likewise, quantitative immunocytochemical measurements using an antibody specific for the PKC-activated Ca2+/GTP-binding protein calexcitin showed enhanced antibody labeling with bryostatin. Animals exposed to the PKC inhibitor bisindolylmaleimide-XI (Ro-32-0432) administered by immersion prior to 9-TE conditioning showed no training-induced changes with or without bryostatin exposure. However, if animals received bryostatin before Ro-32, the enhanced acquisition and demonstrated recall still occurred. Therefore, pathways responsible for the enhancement effects induced by bryostatin were putatively mediated by PKC. Overall, the data indicated that PKC activation occurred and calexcitin levels were raised during the acquisition phases of associative conditioning and memory initiation, and subsequently returned to baseline levels within 24 and 48 h, respectively. Therefore, the protracted recall measured by the testing regime used was probably due to bryostatin-induced changes during the acquisition and facilitated storage of memory, and not necessarily to enhanced recall of the stored memory when tested many days after training.

Structure of the Neuronal Protein Calexcitin Suggests a Mode of Interaction in Signalling Pathways of Learning and Memory

The three-dimensional structure of the neuronal calcium-sensor protein calexcitin from Loligo pealei has been determined by X-ray analysis at a resolution of 1.8A. Calexcitin is up-regulated following Pavlovian conditioning and has been shown to regulate potassium channels and the ryanodine receptor. Thus, calexcitin is implicated in neuronal excitation and plasticity. The overall structure is predominantly helical and compact with a pronounced hydrophobic core between the N and C-terminal domains of the molecule. The structure consists of four EF-hand motifs although only the first three EF hands are involved in binding calcium ions; the C-terminal EF-hand lacks the amino acids required for calcium binding. The overall structure is quite similar to that of the sarcoplasmic calcium-binding protein from Amphioxus although the sequence identity is very low at 31%. The structure shows that the two amino acids of calexcitin phosphorylated by protein kinase C are close to the domain interface in three dimensions and thus phosphorylation is likely to regulate the opening of the domains that is probably required for binding to target proteins. There is evidence that calexcitin is a GTPase and the residues, which have been implicated by mutagenesis in its GTPase activity, are in a short but highly conserved region of 3(10) helix close to the C terminus. This helix resides in a large loop that is partly sandwiched between the N and C-terminal domains suggesting that GTP binding may also require or may cause domain opening. The structure possesses a pronounced electropositive crevice in the vicinity of the 3(10) helix, that might provide an initial docking site for the triphosphate group of GTP. These findings elucidate a number of the reported functions of calexcitin with implications for neuronal signalling.

Crystallization and preliminary X-ray diffraction analysis of calexcitin from Loligo pealei: a neuronal protein implicated in learning and memory

The neuronal protein calexcitin from the long-finned squid Loligo pealei has been expressed in Escherichia coli and purified to homogeneity. Calexcitin is a 22 kDa calcium-binding protein that becomes up-regulated in invertebrates following Pavlovian conditioning and is likely to be involved in signal transduction events associated with learning and memory. Recombinant squid calexcitin has been crystallized using the hanging-drop vapour-diffusion technique in the orthorhombic space group P2(1)2(1)2(1). The unit-cell parameters of a = 46.6, b = 69.2, c = 134.8 A suggest that the crystals contain two monomers per asymmetric unit and have a solvent content of 49%. This crystal form diffracts X-rays to at least 1.8 A resolution and yields data of high quality using synchrotron radiation.

Pavlovian conditioning of Hermissenda: current cellular, molecular, and circuit perspectives

The less-complex central nervous system of many invertebrates make them attractive for not only the molecular analysis of the associative learning and memory, but also in determining how neural circuits are modified by learning to generate changes in behavior. The nudibranch mollusk Hermissenda crassicornis is a preparation that has contributed to an understanding of cellular and molecular mechanisms of Pavlovian conditioning. Identified neurons in the conditioned stimulus (CS) pathway have been studied in detail using biophysical, biochemical, and molecular techniques. These studies have resulted in the identification and characterization of specific membrane conductances contributing to enhanced excitability and synaptic facilitation in the CS pathway of conditioned animals. Second-messenger systems activated by the CS and US have been examined, and proteins that are regulated by one-trial and multi-trial Pavlovian conditioning have been identified in the CS pathway. The recent progress that has been made in the identification of the neural circuitry supporting the unconditioned response (UR) and conditioned response (CR) now provides for the opportunity to understand how Pavlovian conditioning is expressed in behavior.

Orchestration of synaptic plasticity through AKAP signaling complexes

Significant progress has been made toward understanding the mechanisms by which organisms learn from experiences and how those experiences are translated into memories. Advances in molecular, electrophysiological and genetic technologies have permitted great strides in identifying biochemical and structural changes that occur at synapses during processes that are thought to underlie learning and memory. Cellular events that generate the second messenger cyclic AMP (cAMP) and activate protein kinase A (PKA) have been linked to synaptic plasticity and long-term memory. In this review we will focus on the role of PKA in synaptic plasticity and discuss how the compartmentalization of PKA through its association with A-Kinase Anchoring Proteins (AKAPs) affect PKA function in this process.

Parallel processing in an identified neural circuit: the Aplysia californica gill-withdrawal response model system

The response of the gill of Aplysia calfornica Cooper to weak to moderate tactile stimulation of the siphon, the gill-withdrawal response or GWR, has been an important model system for work aimed at understanding the relationship between neural plasticity and simple forms of non-associative and associative learning. Interest in the GWR has been based largely on the hypothesis that the response could be explained adequately by parallel monosynaptic reflex arcs between six parietovisceral ganglion (PVG) gill motor neurons (GMNs) and a cluster of sensory neurons termed the LE cluster. This hypothesis, the Kupfermann-Kandel model, made clear, falsifiable predictions that have stimulated experimental work for many years. Here, we review tests of three predictions of the Kupfermann-Kandel model: (1) that the GWR is a simple, reflexive behaviour graded with stimulus intensity; (2) that central nervous system (CNS) pathways are necessary and sufficient for the GWR; and (3) that activity in six identified GMNs is sufficient to account for the GWR. The available data suggest that (1) a variety of action patterns occur in the context of the GWR; (2) the PVG is not necessary and the diffuse peripheral nervous system (PNS) is sufficient to mediate these action patterns; and (3) the role of any individual GMN in the behaviour varies. Both the control of gill-withdrawal responses, and plasticity in these responses, are broadly distributed across both PNS and CNS pathways. The Kupfermann-Kandel model is inconsistent with the available data and therefore stands rejected. There is, no known causal connection or correlation between the observed plasticity at the identified synapses in this system and behavioural changes during non-associative and associative learning paradigms. Critical examination of these well-studied central pathways suggests that they represent a 'wetware' neural network, architecturally similar to the neural network models of the widely used 'Perceptron' and/or 'Back-propagation' type. Such models may offer a more biologically realistic representation of nervous system organisation than has been thought. In this model, the six parallel GMNs of the CNS correspond to a hidden layer within one module of the gill-control system. That is, the gill-control system appears to be organised as a distributed system with several parallel modules, some of which are neural networks in their own right. A new model is presented here which predicts that the six GMNs serve as components of a 'push-pull' gain control system, along with known but largely unidentified inhibitory motor neurons from the PVG. This 'push-pull' gain control system sets the responsiveness of the peripheral gill motor system. Neither causal nor correlational links between specific forms of neural plasticity and behavioural plasticity have been demonstrated in the GWR model system. However, the GWR model system does provide an opportunity to observe and describe directly the physiological and biochemical mechanisms of distributed representation and parallel processing in a largely identifiable 'wetware' neural network.

Status of the "protein kinase CK2-HMG14" system in age-related amnesia in rats

The experiments described here demonstrate that disruption of the phosphorylation of transcription factors of the HMG cAMP/Ca-independent protein kinase CK2 class may be the cause of decreased gene expression in age-related cognitive deficits. Amnesia for a conditioned passive avoidance reaction (CPAR) in aged rats (24 months old) was accompanied by decreases in the synthesis of synaptosomal proteins and transcription in nuclei isolated from cortical, hippocampal, and striatal neurons. There was a decrease in chromatin protein kinase CK2 activity and a significant decrease in the phosphorylation of HMG14 by protein kinase CK2. Selective activators of protein kinase CK2 (1-ethyl-4-carbamoyl-5-methylcarbamoylimidazole and 1-ethyl-4,5-dicarbamoylimidazole) increased HMG14 phosphorylation by protein kinase CK2, increased transcription, increased the synthesis of synaptosomal proteins, and decreased amnesia for the CPAR in aged rats. Thus, activation of the "protein kinase CK2-HMG14" system is accompanied by optimization of synaptic plasticity in aged animals. The results provide evidence for the high therapeutic potential of protein kinase CK2 activators.

Calcium-regulated GTPase activity in the calcium-binding protein calexcitin

Calexcitin (CE) is a calcium-binding protein, closely related to sarcoplasmic calcium-binding proteins, that is involved in invertebrate learning and memory. Early reports indicated that both Hermissenda and squid CE also could bind GTP; however, the biochemical significance of GTP-binding and its relationship to calcium binding have remained unclear. Here, we report that the GTPase activity of CE is strongly regulated by calcium. CE possessed a P-loop-like structure near the C-terminal similar to the phosphate-binding regions in other GTP-binding proteins. Site-directed mutagenesis of this region showed that Gly(182), Phe(186) and Gly(187) are required for maximum affinity, suggesting that the GTP-binding motif is G-N-x-x-[FM]-G. CE cloned from Drosophila CNS possessed a similar C-terminal sequence and also bound and hydrolyzed GTP. GTPase activity in Drosophila CE was also strongly regulated by Ca(2+), exhibiting over 23-fold higher activity in the presence of 0.3 microM calcium. Analysis of the conserved protein motifs defines a new family of Ca(2+)-binding proteins representing the first example of proteins endowed with both EF-hand calcium binding domains and a C-terminal, P-loop-like GTP-binding motif. These results establish that, in the absence of calcium, both squid and Drosophila CE bind GTP at near-physiological concentrations and hydrolyze GTP at rates comparable to unactivated ras. Calcium functions to increase GTP-binding and GTPase activity in CE, similar to the effect of GTPase activating proteins in other low-MW GTP-binding proteins. CE may, therefore, act as a molecular interface between Ca(2+) cytosolic oscillations and the G protein-coupled signal transduction.

AMPA receptor trafficking and long-term potentiation

Activity-dependent changes in synaptic function are believed to underlie the formation of memories. A prominent example is long-term potentiation (LTP), whose mechanisms have been the subject of considerable scrutiny over the past few decades. I review studies from our laboratory that support a critical role for AMPA receptor trafficking in LTP and experience-dependent plasticity.

AMPA receptor trafficking and synaptic plasticity

Activity-dependent changes in synaptic function are believed to underlie the formation of memories. Two prominent examples are long-term potentiation (LTP) and long-term depression (LTD), whose mechanisms have been the subject of considerable scrutiny over the past few decades. Here we review the growing literature that supports a critical role for AMPA receptor trafficking in LTP and LTD, focusing on the roles proposed for specific AMPA receptor subunits and their interacting proteins. While much work remains to understand the molecular basis for synaptic plasticity, recent results on AMPA receptor trafficking provide a clear conceptual framework for future studies.

Studies of the relationship between ultrastructural synaptic plasticity and ribosome number in dendritic terminals in the rat neocortex in a cellular conditioning model

The relationship between structural changes in postsynaptic densities of axodendritic synapses and the sizes of postsynaptic ribosomal aggregations were studied. A positive correlation was found between the thickness of the postsynaptic density and the number of ribosomes. The role of dendritic mRNA and the possible mechanisms supporting rapid local protein synthesis during the modification of postsynaptic components is seen on combined administration of two neuromediators into the rat neocortex.

Involvement of the genetic apparatus in memory formation mechanisms: the role of the neuronal calcium-regulatory system in rats

Stimulators of long-term memory (ethylnorantiphein and its analogs M1 and M2) were used to study the dynamics of several components of the neuronal calcium-regulatory system in the rat cortex and hippocampus. There were no changes in the activity of the Mg, Ca-ATPase transporter and actomyosin-like Ca-ATPase in synaptosomes 5, 15, 60, and 180 min after dosage with these agents. On exposure to ethylnorantiphein, M1, and M2, activation of RNA transcription at 60 min was accompanied by notable increases in chromatin Ca-ATPase activity, along with an increase in the synthesis of synaptosomal proteins at 180 min, with an increase in synaptic membrane protein kinase C activity. An increase in chromatin Ca-ATPase activity was also seen during fixation of a conditioned active escape reflex. It is suggested that the increase in protein kinase C activity is associated with secondary rearrangements of the synaptic membranes. The question of the role of direct activation of the genetic apparatus by neuroactive substances in the molecular mechanisms of memory formation is discussed.

Chelerythrine, a specific PKC inhibitor, blocks acquisition but not consolidation and retrieval of conditioned taste aversion in rat

Association of the short-term memory of the gustatory conditioned stimulus (CS) with visceral malaise (unconditioned stimulus, US) in conditioned taste aversion (CTA) paradigm takes place in the parabrachial nuclei (PBN) of brainstem. In order to ascertain the role of protein-kinase C (PKC) during different phases of CTA acquisition and retrieval, four experimental series were carried out. In Experiment 1, 1 microl of 10 mM of PKC inhibitor chelerythrine prevented CTA acquisition when applied into PBN in the CS-US interval. In Experiment 2, the necessity of PKC activity in different phases of CTA acquisition was tested by prolonging the time interval between PBN administration of chelerythrine and i.p. LiCl. CTA acquisition was prevented when chelerythrine-induced blockade of PKC coincided with GSTM persistence but not with CTA consolidation. In Experiment 3, the interval between saccharin drinking and LiCl injection was prolonged to 120 min. Again, chelerythrine blockade of PKC activity prevented CTA formation when it interfered with GSTM persistence. In Experiment 4, the possibility that PKC activity is necessary also for CTA retrieval was tested by chelerythrine application into PBN 5 min before retrieval testing. In this case, the chelerythrine-induced PKC blockade did not impair CTA retrieval. It is concluded that PKC is important for GSTM formation and persistence but not for CTA consolidation or retrieval.

Secondary structure and Ca2+-induced conformational change of calexcitin, a learning-associated protein

Calexcitin/cp20 is a low molecular weight GTP- and Ca2+-binding protein, which is phosphorylated by protein kinase C during associative learning, and reproduces many of the cellular effects of learning, such as the reduction of potassium currents in neurons. Here, the secondary structure of cloned squid calexcitin was determined by circular dichroism in aqueous solution and by Fourier transform infrared spectroscopy both in solution and on dried films. The results obtained with the two techniques are in agreement with each other and coincide with the secondary structure computed from the amino acid sequence. In solution, calexcitin is one-third in alpha-helix and one-fifth in beta-sheet. The conformation of the protein in solid state depends on the concentration of the starting solution, suggesting the occurrence of surface aggregation. The secondary structure also depends on the binding of calcium, which causes an increase in alpha-helix and a decrease in beta-sheet, as estimated by circular dichroism. The conformation of calexcitin is independent of ionic strength, and the calcium-induced structural transition is slightly inhibited by Mg2+ and low pH, while favored by high pH. The switch of calexcitin's secondary structure upon calcium binding, which was confirmed by intrinsic fluorescence spectroscopy and nondenaturing gel electrophoresis, is reversible and occurs in a physiologically meaningful range of Ca2+ concentration. The calcium-bound form is more globular than the apoprotein. Unlike other EF-hand proteins, calexcitin's overall lipophilicity is not affected by calcium binding, as assessed by hydrophobic liquid chromatography. Preliminary results from patch-clamp experiments indicated that calcium is necessary for calexcitin to inhibit potassium channels and thus to increase membrane excitability. Therefore the calcium-dependent conformational equilibrium of calexcitin could serve as a molecular switch for the short term modulation of neuronal activity following associative conditioning.

Suppression of c-fos induction in rat brain impairs retention of a brightness discrimination reaction

Recently, the induction of transcription factor-encoding immediate-early genes such as c-fos was observed in distinct brain regions of rats trained to acquire a footshock-motivated brightness discrimination in a Y-maze. The functional relevance of inducible transcription factors for learning and memory formation is, however, not clear. To address this question in the present study, we have used a synthetic antisense phosphorothioate oligodeoxynucleotide to suppress in vivo the expression of c-fos in rat brain. Intrahippocampal application of the oligodeoxynucleotide 10 hr and 2 hr before starting a brightness discrimination training drastically reduced the induction of c-Fos immunoreactivity normally observed in limbic and cortical areas after the training session. Acquisition of the discrimination reaction was not affected by this treatment. In a relearning test 24 hr after the first training, retention of the discrimination reaction was specifically impaired compared with rats pretreated with control oligodeoxynucleotide or saline. Our findings are consistent with the hypothesis that the inducible transcription factor c-Fos is involved in processes underlying the formation of long-term memory.

Calexcitin: a signaling protein that binds calcium and GTP, inhibits potassium channels, and enhances membrane excitability

A previously uncharacterized 22-kDa Ca(2+)-binding protein that also binds guanosine nucleotides was characterized, cloned, and analyzed by electrophysiological techniques. The cloned protein, calexcitin, contains two EF-hands and also has homology with GTP-binding proteins in the ADP ribosylation factor family. In addition to binding two molecules of Ca2+, calexcitin bound GTP and possessed GTPase activity. Calexictin is also a high affinity substrate for protein kinase C. Application of calexcitin to the inner surface of inside-out patches of human fibroblast membranes, in the presence of Ca2+ and the absence of endogenous Ca2+/calmodulin kinase type II or protein kinase C activity, reduced the mean open time and mean open probability of 115 +/- 6 pS K+ channels. Calexcitin thus appears to directly regulate K+ channels. When microinjected into molluscan neurons or rabbit cerebellar Purkinje cell dendrites, calexcitin was highly effective in enhancing membrane excitability. Because calexcitin translocates to the cell membrane after phosphorylation, calexcitin could serve as a Ca(2+)-activated signaling molecule that increases cellular excitability, which would in turn increase Ca2+ influx through the membrane. This is also the first known instance of a GTP-binding protein that binds Ca2+.

Protein kinases in the locus coeruleus and periaqueductal gray matter are involved in the expression of opiate withdrawal

The aim of this study was to evaluate the role played in the behavioral expression of morphine withdrawal syndrome by protein kinases in the locus coeruleus and the periaqueductal gray matter. Two different families of specific protein kinases have been investigated: serine/threonine and tyrosine kinases. Rats were implanted with cannulas into both the lateral ventricle and the locus coeruleus or the periaqueductal gray matter. Physical dependence was induced by chronic peripheral administration of morphine (from 7 to 30 mg/kg) and withdrawal syndrome was precipitated by injection of naloxone (2 micrograms) into the lateral ventricle. The administration of the serine/threonine kinase inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine, H7 (1, 3, 10, and 30 nmol per side) into the locus coeruleus induced a strong attenuation of morphine withdrawal behavior. Signs related to the motor component of abstinence, such as jumping, rearing, and hyperactivity, were the most severely reduced. However, this effect was not dose-dependent, and the response was almost the same with all the doses used. A similar attenuation was observed after the injection of H7 (1, 3, and 10 nmol per side) into the periaqueductal gray matter, but in this case motor signs were less strongly reduced and a larger number of signs were modified, mainly when using the highest dose. The administration of the tyrosine kinase inhibitor 2-hydroxy-5-[N(2,5-dihydroxyphenyl)methyl]amino]-benzoic acid 3-phenylpropyl ester, KB23 (0.3, 1, and 3 nmol per side) into the locus coeruleus or the periaqueductal gray matter had no effect on the withdrawal syndrome behavior, except on teeth chattering. These results suggest that in the locus coeruleus and in the periaqueductal gray matter, serine/threonine kinases are implicated in the behavioral expression of morphine abstinence. In these brain structures, tyrosine kinases appear not to be involved.

Differential time courses of c-fos mRNA expression in hippocampal subfields following acquisition and recall testing in mice

Spatio-temporal patterns of c-fos mRNA expression were studied in the mouse brain following the partial acquisition of an appetitive conditioning task in a Skinner box. We used two experimental situations: during the initial acquisition of the task (acquisition paradigm) and during the retention test (recall paradigm). In both paradigms the in situ hybridization signal was exclusively located in the hippocampal formation and the posterior cingulate cortex. However, the time-dependent pattern of expression was quite different according to the experimental situation: mRNA levels peaked at 90 min post-test in both paradigms but returned to basal (control) level by 180 min in the acquisition group, while in CA3 and DG subfields, high levels of mRNA expression were maintained at 180 min in the recall group. Taken together these results suggest that the IEG c-fos is implicated in the different phases of post-acquisition memory processes and involve a differential spatio-temporal regulation of its expression in hippocampal subfields.

Repetitive treatment with serotonin modifies protein synthesis and protein phosphorylation in the central nervous system of Hirudo medicinalis

Serotonin (5HT) is the neurotransmitter involved in some forms of short-term memory in the leech. Behavioral experiments have demonstrated that long-term memory requires new protein synthesis. With the aim of studying the molecular mechanism underlying memory processes in the leech, we have analyzed the effect of 5HT on protein synthesis and protein phosphorylation. Segmental ganglia of the leech central nervous system have been labeled, proteins have been separated by two-dimensional-electrophoresis and labeled proteins detected by autoradiography. Our findings indicate that repetitive treatment with 5HT produces either the persistence of phosphorylation or changes in protein synthesis in several proteins.

Ketamine: acquisition and retention of classically conditioned responses during treatment with large doses

Two experiments were conducted in rabbits to examine the effects of ketamine (0, 100, and 200 mg/kg) on the acquisition and retention of the classically conditioned nictitating membrane response (NMR). Classical conditioning of the NMR was accomplished by pairing tone and light conditioned stimuli (CS) with paraorbital shock as the unconditioned stimulus (UCS). Experiment 1 assessed the effects of the drug on acquisition and retention of conditioned responses (CR) and determined the role of previous exposure to the experimental environment. Ketamine blocked the display of CR. However, data from subsequent retention testing under nondrug conditions revealed that rabbits that had previously received 100 mg/kg ketamine learned faster than saline-treated rabbits during the acquisition phases. Rabbits that received 100 mg/kg ketamine and were placed in the experimental chambers, but not presented with stimuli during the acquisition phase, did not learn faster during the retention phase than naive rabbits. Experiment 2 controlled further for the effects of nonassociative, unlearned processes. Control groups were presented with unpaired CS and UCS training after drug administration, and subsequently received conventional acquisition sessions under nondrug conditions. Their data indicated that the ketamine group's rapid acquisition during retention testing could not be attributed to nonassociative factors. We conclude that, although it was impossible directly to observe acquisition in rabbits under the influence of ketamine, it was possible that learning occurred as manifested by "savings" in subsequent learning trials.

Cholesterol treatment facilitates spatial learning performance in DBA/2Ibg mice

DBA/2Ibg mice were treated with cholesterol pellets for 11 days. On the seventh day after treatment, animals began 5 consecutive days of training on the spatial form of Morris water task, followed on the third and fourth days by a probe trial, and random platform training on the fifth day. DBA mice with cholesterol pellets exhibited enhanced performance compared to DBA mice that underwent a sham surgery. Our results suggest that subchronic treatment with the steroid hormone precursor, cholesterol, enhances spatial learning performance in DBA mice.

Integrative activity of the neuron: the specific characteristics and plasticity of systemic mechanisms

The notions of P. K. Anokhin regarding the integrative activity of neurons through the time course of the systemic organization of behavioral acts are developed in this article. It is shown that in the systemic organization of behavioral acts neurons in their activity not only initial motivations and reinforcement, but demonstrate anticipatory reactions on the basis of preceding reinforcements, including themselves in the formation of the apparatus of the acceptor of the result of an action. A dominant motivation substantially alters the sensitivity of brain neurons to neuromediators and neuropeptides. Change in the chemical sensitivity of neurons has been identified through the time course of the organization of the various stages of systemic organization of behavioral acts. It has been established that the sensitivity of neurons to motivational and sensory influences, as well as to biologically active substances is also determined to a substantial degree by their protein-synthesizing apparatus. Investigations have demonstrated the specific characteristics and plasticity of neurons in the systemic organization of behavioral acts.

Blockade of neurotensin-induced motor activity by inhibition of protein kinase

The administration of neurotensin into the ventral tegmental area stimulates dopamine neurons and locomotor activity. Furthermore, when neurotensin is microinjected daily into the ventral tegmental area the motor stimulant response increases. The role of protein kinases in the motor stimulant effect of neurotensin was evaluated by coadministration of the protein kinase inhibitors H8 and H7 into the ventral tegmental area with neurotensin. It was found that the acute motor stimulant effect of neurotensin was abolished in a dose-dependent fashion by H8 coadministration. Neurotensin-induced activity was also blocked by H7. However, acute motor stimulation following microinjection of the mu opioid, Tyr-d-Ala-Gly-MePhe-Gly(ol) or the potassium channel antagonist apamin into the ventral tegmental area was not affected by coadministration with H8. The behavioral sensitization produced by daily neurotensin microinjection into the ventral tegmental area was also prevented by the coadministration of H8. These data indicate that the motor stimulation produced by acute and repeated neurotensin microinjection into the ventral tegmental area is dependent upon activation of protein kinase(s). Furthermore, Tyr-d-Ala-Gly-MePhe-Gly(ol) and apamine elicit locomotion independently of protein kinase(s).

Environmental stimulation promotes changes in the distribution of phorbol ester receptors

The translocation of protein kinase C (PKC) from the cytosol to the membrane might be functionally involved in learning and memory. Using [3H]-phorbol 12,13-dibutyrate (3H-PDBu) binding three pools of binding sites could be distinguished in tissue preparations: Pool a comprised the soluble receptors which bound phorbol ester with low affinity in the absence of calcium. Pool b was composed of high-affinity phorbol ester binding sites identified in the soluble fraction upon addition of calcium. Pool c represented stably membrane-bound receptors binding phorbol ester independently of calcium. 3H-PDBu binding was then measured in the cortices and hippocampi of rats trained in an eight-arm radial maze. A progressive training-dependent increase of membrane-bound binding activity with a concomitant decrease in the soluble fraction was detected independent of learning the maze task. These results suggest that it is the experience of an enriched environment by the repeated behavioral stimulation in a maze rather than the acquisition of a memory task that leads to enhanced incorporation of phorbol ester receptors (PKC) into the cell membrane.

Effect of serotonin on protein phosphorylation in the central nervous system of the leech Hirudo medicinalis

1. Phosphoproteins of different regions of the Hirudo medicinalis central nervous system have been analysed by means of two-dimensional electrophoresis. 2. Serotonin, 8-Br-cAMP and phorbol 12,13-dibutyrate stimulate phosphorylation of a number of proteins whose isoelectric points and molecular weights are presented. 3. A group of proteins of 78 kDa and pI = 6-6.5, whose level of phosphorylation increases in the presence of serotonin, 8-Br-cAMP and phorbol ester, is observed only in segmental but not in cephalic or caudal ganglia. 4. The putative roles of these phosphoproteins are discussed.

Distribution of c-fos expression in brainstem neurons associated with conditioning and pseudo-conditioning of the rabbit nictitating membrane reflex

Experiments were performed to test the hypothesis that there is a characteristic distribution of neuronal c-fos expression associated with the classical conditioning of the rabbit nictitating membrane reflex (NMR). Rabbits were divided into two groups: a conditioning group that received paired tone and airpuff stimuli in a traditional delay NMR conditioning paradigm and a pseudo-conditioning group in which the same number of tone and airpuff stimuli were applied but without being paired. Labeling was present in similar brainstem nuclei in both groups of animals. The labeled sites included trigeminal and auditory nuclei in the classical pathway for the nictitating membrane reflex as well as other nuclei such as the raphe nuclei and those in the ventrolateral medulla (VLM). However, there were quantitative differences in the labeling between the two groups. There were significantly more labeled nuclear profiles in the trigeminal nucleus of the pseudoconditioned rabbits, but more labeled nuclear profiles in the raphe nuclei in the conditioned animals. Interestingly, the ratio of the labeling in the raphe versus the VLM strongly differed between the two groups.

Brief exposure to an enriched environment improves performance on the Morris water task and increases hippocampal cytosolic protein kinase C activity in young rats

This study was designed to determine whether brief exposure to an enriched environment around the time of weaning would affect learning and memory processes in young rats. In addition, this study sought to determine if experience in an enriched environment would alter hippocampal protein kinase C (PKC) which is thought to be a possible neural substrate that underlies learning and memory processes. Animals were either reared in an enriched environment or standard laboratory cages starting at 15 days old. After 6 (21 days old) or 12 (27 days old) days subjects were either tested in the Morris water task, or had the hippocampus removed for biochemical analysis of PKC activity. Morris water task results showed that compared to laboratory reared controls, the performance of subjects reared in the enriched environment for 12 days, but not 6 days, was improved. In addition, 12 days of exposure to the enriched environment, but not 6 days, produced more cytosolic hippocampal PKC activity. The particulate fraction appeared not to be affected by rearing in the enriched environment. Brief exposure to an enriched environment around weaning, therefore, both improved Morris water task performance and increased hippocampal PKC activity. These outcomes suggest that performance in the Morris water task and hippocampal PKC may be functionally related.

Protein synthesis in the hippocampus associated with memory facilitation by corticotropin-releasing factor in rats

The present study used pharmacological, biochemical, and behavioral methods to examine the role of protein synthesis in the hippocampus in memory processes of a passive avoidance learning in rats. Results indicated that corticotropin-releasing factor (CRF) significantly improved memory retention in rats. Both cycloheximide (CHX) and actinomycin-D (ACT-D) impaired memory at high doses. At doses of CHX and ACT-D that did not affect memory alone, they both antagonized the memory-enhancing effect of CRF. Biochemically, there were specific increases in the optical density of three protein bands in the cytosolic fraction of hippocampal cells in rats showing good memory. There were also marked increases in the optical density of two protein bands in the nucleus fraction of the same animals. Similar results were observed in animals injected with CRF. However, no significant protein alteration was observed in animals receiving stress. These results together suggest that there are new protein syntheses in the hippocampus that are specifically associated with passive avoidance learning in rats.

Calcium as sculptor and destroyer of neural circuitry

This paper examines the hypothesis that intracellular calcium plays guiding roles in the formation and adaptive modification of neural circuits in development and adult plasticity and that imbalances in calcium regulation lead to the degeneration of neural circuits in aging and disease. The neuronal growth cone is the motile structure largely responsible for the generation of neuroarchitecture. Studies of developing neurons in culture demonstrated that environmental signals believed to play key roles in neural development (i.e., neurotransmitters and growth factors) regulate growth cones by altering neuronal calcium-regulating systems. Different components of neurite outgrowth (i.e., neurite elongation and growth cone motility) are based upon different cytoskeletal systems (microtubules and microfilaments) which are differentially affected by calcium. In addition, cytoskeleton-associated proteins such as tau and microtubule-associated protein 2 (MAP2) are likely candidates for regulation by calcium. "Natural" neuronal death in development may occur as the result of growth factor deficiency or excess excitatory activity leading to sustained elevations in intracellular calcium levels. With aging and in disease, a loss of calcium homeostasis may underlie the aberrant neurodegeneration that occurs. For example, neurons subjected to conditions (e.g., glutamate and beta-amyloid) that cause sustained rises in intracellular calcium exhibit changes in the cytoskeleton similar to those seen in neurofibrillary tangles of Alzheimer's disease and related disorders. Taken together, the data suggest that cellular systems for calcium homeostasis are integral to both the adaptive and aberrant neuroarchitectural changes that occur throughout the lifespan of the nervous system.

Classical conditioning-induced changes in low-molecular-weight GTP-binding proteins in rabbit hippocampus

Classical conditioning of Hermissenda, involving paired light-rotation events, results in a 30-35% decrease in the levels of a 20-kDa G protein (cp20). To test whether a similar protein exists in vertebrates, rabbits were trained to associate a tone with periorbital electrical stimulation and G proteins were analyzed by photoaffinity labeling with [alpha-32P]GTP-azidoanilide. A 20-kDa G protein similar to cp20 decreased by 36% in the hippocampus of rabbits subjected to paired tone and electrical stimulation, but not in unpaired controls. Learning-specific decreases were also found in the amount of ras protein.