Differential expression of immediate early genes after hippocampal long-term potentiation in awake rats

Age, sex, and apolipoprotein E isoform alter contextual fear learning, neuronal activation, and baseline DNA damage in the hippocampus

Age, female sex, and apolipoprotein E4 (E4) are risk factors to develop Alzheimer's disease (AD). There are three major human apoE isoforms: E2, E3, and E4. Compared to E3, E4 increases while E2 decreases AD risk. However, E2 is associated with increased risk and severity of post-traumatic stress disorder (PTSD). In cognitively healthy adults, E4 carriers have greater brain activation during learning and memory tasks in the absence of behavioral differences. Human apoE targeted replacement (TR) mice display differences in fear extinction that parallel human data: E2 mice show impaired extinction, mirroring heightened PTSD symptoms in E2 combat veterans. Recently, an adaptive role of DNA double strand breaks (DSBs) in immediate early gene expression (IEG) has been described. Age and disease synergistically increase DNA damage and decrease DNA repair. As the mechanisms underlying the relative risks of apoE, sex, and their interactions in aging are unclear, we used young (3 months) and middle-aged (12 months) male and female TR mice to investigate the influence of these factors on DSBs and IEGs at baseline and following contextual fear conditioning. We assessed brain-wide changes in neural activation following fear conditioning using whole-brain cFos imaging in young female TR mice. E4 mice froze more during fear conditioning and had lower cFos immunoreactivity across regions important for somatosensation and contextual encoding compared to E2 mice. E4 mice also showed altered co-activation compared to E3 mice, corresponding to human MRI and cognitive data, and indicating that there are differences in brain activity and connectivity at young ages independent of fear learning. There were increased DSB markers in middle-aged animals and alterations to cFos levels dependent on sex and isoform, as well. The increase in hippocampal DSB markers in middle-aged animals and female E4 mice may play a role in the risk for developing AD.

Secreted Amyloid Precursor Protein Alpha (sAPPα) Regulates the Cellular Proteome and Secretome of Mouse Primary Astrocytes

Secreted amyloid precursor protein alpha (sAPPα), processed from a parent mammalian brain protein, amyloid precursor protein, can modulate learning and memory. Recently it has been shown to modulate the transcriptome and proteome of human neurons, including proteins with neurological functions. Here, we analysed whether the acute administration of sAPPα facilitated changes in the proteome and secretome of mouse primary astrocytes in culture. Astrocytes contribute to the neuronal processes of neurogenesis, synaptogenesis and synaptic plasticity. Cortical mouse astrocytes in culture were exposed to 1 nM sAPPα, and changes in both the whole-cell proteome (2 h) and the secretome (6 h) were identified with Sequential Window Acquisition of All Theoretical Fragment Ion Spectra-Mass Spectrometry (SWATH-MS). Differentially regulated proteins were identified in both the cellular proteome and secretome that are involved with neurologically related functions of the normal physiology of the brain and central nervous system. Groups of proteins have a relationship to APP and have roles in the modulation of cell morphology, vesicle dynamics and the myelin sheath. Some are related to pathways containing proteins whose genes have been previously implicated in Alzheimer's disease (AD). The secretome is also enriched in proteins related to Insulin Growth Factor 2 (IGF2) signaling and the extracellular matrix (ECM). There is the promise that a more specific investigation of these proteins will help to understand the mechanisms of how sAPPα signaling affects memory formation.

Remembering and forgetting in sleep: Selective synaptic plasticity during sleep driven by scaling factors Homer1a and Arc

Sleep is a conserved and essential process that supports learning and memory. Synapses are a major target of sleep function and a locus of sleep need. Evidence in the literature suggests that the need for sleep has a cellular or microcircuit level basis, and that sleep need can accumulate within localized brain regions as a function of waking activity. Activation of sleep promoting kinases and accumulation of synaptic phosphorylation was recently shown to be part of the molecular basis for the localized sleep need. A prominent hypothesis in the field suggests that some benefits of sleep are mediated by a broad but selective weakening, or scaling-down, of synaptic strength during sleep in order to offset increased excitability from synaptic potentiation during wake. The literature also shows that synapses can be strengthened during sleep, raising the question of what molecular mechanisms may allow for selection of synaptic plasticity types during sleep. Here I describe mechanisms of action of the scaling factors Arc and Homer1a in selective plasticity and links with sleep need. Arc and Homer1a are induced in neurons in response to waking neuronal activity and accumulate with time spent awake. I suggest that during sleep, Arc and Homer1a drive broad weakening of synapses through homeostatic scaling-down, but in a manner that is sensitive to the plasticity history of individual synapses, based on patterned phosphorylation of synaptic proteins. Therefore, Arc and Homer1a may offer insights into the intricate links between a cellular basis of sleep need and memory consolidation during sleep.

An Adaptive Role for DNA Double-Strand Breaks in Hippocampus-Dependent Learning and Memory

DNA double-strand breaks (DSBs), classified as the most harmful type of DNA damage based on the complexity of repair, lead to apoptosis or tumorigenesis. In aging, DNA damage increases and DNA repair decreases. This is exacerbated in disease, as post-mortem tissue from patients diagnosed with mild cognitive impairment (MCI) or Alzheimer's disease (AD) show increased DSBs. A novel role for DSBs in immediate early gene (IEG) expression, learning, and memory has been suggested. Inducing neuronal activity leads to increases in DSBs and upregulation of IEGs, while increasing DSBs and inhibiting DSB repair impairs long-term memory and alters IEG expression. Consistent with this pattern, mice carrying dominant AD mutations have increased baseline DSBs, and impaired DSB repair is observed. These data suggest an adaptive role for DSBs in the central nervous system and dysregulation of DSBs and/or repair might drive age-related cognitive decline (ACD), MCI, and AD. In this review, we discuss the adaptive role of DSBs in hippocampus-dependent learning, memory, and IEG expression. We summarize IEGs, the history of DSBs, and DSBs in synaptic plasticity, aging, and AD. DSBs likely have adaptive functions in the brain, and even subtle alterations in their formation and repair could alter IEGs, learning, and memory.

Infusion of etoposide in the CA1 disrupts hippocampal immediate early gene expression and hippocampus-dependent learning

Tight regulation of immediate early gene (IEG) expression is important for synaptic plasticity, learning, and memory. Recent work has suggested that DNA double strand breaks (DSBs) may have an adaptive role in post-mitotic cells to induce IEG expression. Physiological activity in cultured neurons as well as behavioral training leads to increased DSBs and subsequent IEG expression. Additionally, infusion of etoposide-a common cancer treatment that induces DSBs-impairs trace fear memory. Here, we assessed the effects of hippocampal infusion of 60 ng of etoposide on IEG expression, learning, and memory in 3-4 month-old C57Bl/6J mice. Etoposide altered expression of the immediate early genes cFos and Arc in the hippocampus and impaired hippocampus-dependent contextual fear memory. These data add to the growing evidence that DSBs play an important role in IEG expression, learning, and memory, opening avenues for developing novel treatment strategies for memory-related disorders.

Secreted Amyloid Precursor Protein Alpha, a Neuroprotective Protein in the Brain Has Widespread Effects on the Transcriptome and Proteome of Human Inducible Pluripotent Stem Cell-Derived Glutamatergic Neurons Related to Memory Mechanisms

Secreted amyloid precursor protein alpha (sAPPα) processed from a parent human brain protein, APP, can modulate learning and memory. It has potential for development as a therapy preventing, delaying, or even reversing Alzheimer’s disease. In this study a comprehensive analysis to understand how it affects the transcriptome and proteome of the human neuron was undertaken. Human inducible pluripotent stem cell (iPSC)-derived glutamatergic neurons in culture were exposed to 1 nM sAPPα over a time course and changes in the transcriptome and proteome were identified with RNA sequencing and Sequential Window Acquisition of All THeoretical Fragment Ion Spectra-Mass Spectrometry (SWATH-MS), respectively. A large subset (∼30%) of differentially expressed transcripts and proteins were functionally involved with the molecular biology of learning and memory, consistent with reported links of sAPPα to memory enhancement, as well as neurogenic, neurotrophic, and neuroprotective phenotypes in previous studies. Differentially regulated proteins included those encoded in previously identified Alzheimer’s risk genes, APP processing related proteins, proteins involved in synaptogenesis, neurotransmitters, receptors, synaptic vesicle proteins, cytoskeletal proteins, proteins involved in protein and organelle trafficking, and proteins important for cell signalling, transcriptional splicing, and functions of the proteasome and lysosome. We have identified a complex set of genes affected by sAPPα, which may aid further investigation into the mechanism of how this neuroprotective protein affects memory formation and how it might be used as an Alzheimer’s disease therapy.

Glutamate receptors and synaptic plasticity: The impact of Evans and Watkins

On the occasion of the 40 year anniversary of the hugely impactful review by Richard (Dick) Evans and Jeff Watkins, we describe how their work has impacted the field of synaptic plasticity. We describe their influence in each of the major glutamate receptor subtypes: AMPARs, NMDARs, KARs and mGluRs. Particular emphasis is placed on how their work impacted our own studies in the hippocampus. For example, we describe how the tools and regulators that they identified for studying NMDARs (e.g., NMDA, D-AP5 and Mg²⁺) led to the understanding of the molecular basis of the induction of LTP. We also describe how other tools that they introduced (e.g., (1S,3R)-ACPD and MCPG) helped lead to the concept of metaplasticity.

Amifostine (WR-2721) Mitigates Cognitive Injury Induced by Heavy Ion Radiation in Male Mice and Alters Behavior and Brain Connectivity

The deep space environment contains many risks to astronauts during space missions, such as galactic cosmic rays (GCRs) comprised of naturally occurring heavy ions. Heavy ion radiation is increasingly being used in cancer therapy, including novel regimens involving carbon therapy. Previous investigations involving simulated space radiation have indicated a host of detrimental cognitive and behavioral effects. Therefore, there is an increasing need to counteract these deleterious effects of heavy ion radiation. Here, we assessed the ability of amifostine to mitigate cognitive injury induced by simulated GCRs in C57Bl/6J male and female mice. Six-month-old mice received an intraperitoneal injection of saline, 107 mg/kg, or 214 mg/kg of amifostine 1 h prior to exposure to a simplified five-ion radiation (protons, 28Si, 4He, 16O, and 56Fe) at 500 mGy or sham radiation. Mice were behaviorally tested 2-3 months later. Male mice that received saline and radiation exposure failed to show novel object recognition, which was reversed by both doses of amifostine. Conversely, female mice that received saline and radiation exposure displayed intact object recognition, but those that received amifostine prior to radiation did not. Amifostine and radiation also had distinct effects on males and females in the open field, with amifostine affecting distance moved over time in both sexes, and radiation affecting time spent in the center in females only. Whole-brain analysis of cFos immunoreactivity in male mice indicated that amifostine and radiation altered regional connectivity in areas involved in novel object recognition. These data support that amifostine has potential as a countermeasure against cognitive injury following proton and heavy ion irradiation in males.

Bilateral histone deacetylase 1 and 2 activity and enrichment at unique genes following induction of long-term potentiation in vivo

Long-term potentiation (LTP) is a synaptic plasticity mechanism critical to long-term memory. LTP induced in vivo is characterized by altered transcriptional activity, including a period of upregulation of gene expression which is followed by a later dominant downregulation. This temporal shift to downregulated gene expression is predicted to be partly mediated by epigenetic inhibitors of gene expression, such as histone deacetylases (HDACs). Further, pharmacological inhibitors of HDAC activity have previously been shown to enhance LTP persistence in vitro. To explore the contribution of HDACs to the persistence of LTP in vivo, we examined HDAC1 and HDAC2 activity over a 24 hr period following unilateral LTP induction in the dentate gyrus of freely moving rats. Surprisingly, we found significant changes in HDAC1 and HDAC2 activity in both the stimulated as well as the unstimulated hemispheres, with the largest increase in activity occurring bilaterally, 20 min after LTP stimulation. During this time point of heightened activity, chromatin immunoprecipitation assays showed that both HDAC1 and HDAC2 were enriched at distinct sets of genes within each hemispheres. Further, the HDAC inhibitor Trichostatin A enhanced an intermediate phase of LTP lasting days, which has not previously been associated with altered transcription. The inhibitor had no effect on the persistence of LTP lasting weeks. Together, these data suggest that HDAC activity early after the induction of LTP may negatively regulate plasticity-related gene expression that is involved in the initial stabilization of LTP, but not its long-term maintenance.

The cellular and molecular basis of in vivo synaptic plasticity in rodents

Plasticity within the neuronal networks of the brain underlies the ability to learn and retain new information. The initial discovery of synaptic plasticity occurred by measuring synaptic strength in vivo, applying external stimulation and observing an increase in synaptic strength termed long-term potentiation (LTP). Many of the molecular pathways involved in LTP and other forms of synaptic plasticity were subsequently uncovered in vitro. Over the last few decades, technological advances in recording and imaging in live animals have seen many of these molecular mechanisms confirmed in vivo, including structural changes both pre- and postsynaptically, changes in synaptic strength, and changes in neuronal excitability. A well-studied aspect of neuronal plasticity is the capacity of the brain to adapt to its environment, gained by comparing the brains of deprived and experienced animals in vivo, and in direct response to sensory stimuli. Multiple in vivo studies have also strongly linked plastic changes to memory by interfering with the expression of plasticity and by manipulating memory engrams. Plasticity in vivo also occurs in the absence of any form of external stimulation, i.e., during spontaneous network activity occurring with brain development. However, there is still much to learn about how plasticity is induced during natural learning and how this is altered in neurological disorders.

Electrifying the motor engram: effects of tDCS on motor learning and control

Learning to control our movements is accompanied by neuroplasticity of motor areas of the brain. The mechanisms of neuroplasticity are diverse and produce what is referred to as the motor engram, i.e., the neural trace of the motor memory. Transcranial direct current stimulation (tDCS) alters the neural and behavioral correlates of motor learning, but its precise influence on the motor engram is unknown. In this review, we summarize the effects of tDCS on neural activity and suggest a few key principles: (1) Firing rates are increased by anodal polarization and decreased by cathodal polarization, (2) anodal polarization strengthens newly formed associations, and (3) polarization modulates the memory of new/preferred firing patterns. With these principles in mind, we review the effects of tDCS on motor control, motor learning, and clinical applications. The increased spontaneous and evoked firing rates may account for the modulation of dexterity in non-learning tasks by tDCS. The facilitation of new association may account for the effect of tDCS on learning in sequence tasks while the ability of tDCS to strengthen memories of new firing patterns may underlie the effect of tDCS on consolidation of skills. We then describe the mechanisms of neuroplasticity of motor cortical areas and how they might be influenced by tDCS. We end with current challenges for the fields of brain stimulation and motor learning.

Oxidative stress in synapse development and function

Oxidative stress, caused by increased levels of reactive oxidative species (ROS), is considered a major contributor to the aging process. How oxidative stress may bring about changes to structures and function in the aging brain is poorly understood. Oxidative stress activates a number of cellular responses, including activation of the Jun-N-terminal kinase (JNK) pathway and autophagy. In addition to their pathological role, ROS also act as signaling molecules. ROS such as nitric oxide have a well-known role in learning and memory. In addition, activation of JNK and its transcriptional effector AP-1 are well-known mediators of synaptic function and growth. Both are essential mediators of physiological correlates of learning and memory such as long-term potentiation. JNK and AP-1 are potently activated and regulated by oxidative stress and mediate protective cellular responses such as autophagy. Recent work at the Drosophila neuromuscular junction implicates autophagy as a regulator of synaptic growth via activation of the JNK signaling pathway. We here outline a framework predicating oxidative stress as a major regulator of synaptic function and growth by the activation of JNK/AP-1 and autophagy. Such responses, we suggest, may underpin some forms of synaptic growth responses and synaptic aging.

Rapidly induced gene networks following induction of long-term potentiation at perforant path synapses in vivo

The canonical view of the maintenance of long-term potentiation (LTP), a widely accepted experimental model for memory processes, is that new gene transcription contributes to its consolidation; however, the gene networks involved are unknown. To address this issue, we have used high-density Rat 230.2 Affymetrix arrays to establish a set of genes induced 20-min post-LTP, and using Ingenuity Pathway network analysis tools we have investigated how these early responding genes are interrelated. This analysis identified LTP-induced regulatory networks in which the transcription factors (TFs) nuclear factor-KB and serum response factor, which, to date, have not been widely recognized as coordinating the early gene response, play a key role alongside the more well-known TFs cyclic AMP response element-binding protein, and early growth response 1. Analysis of gene-regulatory promoter sites and chromosomal locations of the genes within the dataset reinforced the importance of these molecules in the early gene response and predicted that the coordinated action might arise from gene clustering on particular chromosomes. We have also identified a transcription-based response that affects mitogen-activated protein kinase signaling pathways and protein synthesis during the stabilization of the LTP response. Furthermore, evidence from biological function, networks, and regulatory analyses showed convergence on genes related to development, proliferation, and neurogenesis, suggesting that these functions are regulated early following LTP induction. This raises the interesting possibility that LTP-related gene expression plays a role in both synaptic reorganization and neurogenesis.

Activation and function of immediate-early genes in the nervous system

Immediate-early genes have important roles in processes such as brain development, learning, and responses to drug abuse. Further, immediate-early genes play an essential role in cellular responses that contribute to long-term neuronal plasticity. Neuronal plasticity is a characteristic of the nervous system that is not limited to the first stages of brain development but persists in adulthood and seems to be an inherent feature of everyday brain function. The plasticity refers to the neuron's capability of showing short- or long-lasting phenotypic changes in response to different stimuli and cellular scenarios. In this review, we focus on the immediate-early genes encoding transcription factors (AP-1 and Egr) that are relevant for neuronal responses. Our current understanding of the mechanisms involved in the induction of the immediate-early genes is presented.

Immediate early gene expression invoked by electrical intracochlear stimulation in some but not all types of neurons in the rat auditory brainstem

Specific patterns of sensory activity may induce plastic remodeling of neurons and the communication network they form in the adult mammalian brain. Among the indicators for the initiation of neuronal remodeling is the expression of immediate early genes (IEGs). The IEGs c-fos and egr-1 encode transcription factors. Following spectrally and temporally precisely defined unilateral electrical intracochlear stimulation (EIS) that corresponded in strength to physiological acoustic stimuli and lasted for 2 h under anesthesia, we characterized those neuronal cell types in ventral (VCN) and dorsal cochlear nucleus (DCN), lateral superior olive (LSO) and central nucleus of the inferior colliculus (CIC) of the rat brain that expressed IEGs. We found that EIS affected only specific types of neurons. Whereas sub-populations of glutamatergic and glycinergic cells responded in all four regions, GABAergic neurons failed to do so except in DCN. Combining immunocytochemistry with axonal tracing, neurons participating in major ascending pathways, commissural cells of VCN and certain types of neurons of the descending auditory system were seen to respond to EIS with IEG expression. By contrast, principal LSO cells projecting to the contralateral CIC as well as collicular efferents of the DCN did not. In total, less than 50% of the identified neurons turned up expression of the IEGs studied. The pattern of IEG expression caused by unilateral EIS led us to suggest that dominant sensory activity may quickly initiate a facilitation of central pathways serving the active ear at the expense of those serving the unstimulated ear.

Long-term depression activates transcription of immediate early transcription factor genes: involvement of serum response factor/Elk-1

Long-term depression (LTD) is one of the paradigms used in vivo or ex vivo for studying memory formation. In order to identify genes with potential relevance for memory formation we used mouse organotypic hippocampal slice cultures in which chemical LTD was induced by applications of 3,5-dihydroxyphenylglycine (DHPG). The induction of chemical LTD was robust, as monitored electrophysiologically. Gene expression analysis after chemical LTD induction was performed using cDNA microarrays containing >7,000 probes. The DHPG-induced expression of immediate early genes (c-fos, junB, egr1 and nr4a1) was subsequently verified by TaqMan polymerase chain reaction. Bioinformatic analysis suggested a common regulator element [serum response factor (SRF)/Elk-1 binding sites] within the promoter region of these genes. Indeed, here we could show a DHPG-dependent binding of SRF at the SRF response element (SRE) site within the promoter region of c-fos and junB. However, SRF binding to egr1 promoter sites was constitutive. The phosphorylation of the ternary complex factor Elk-1 and its localization in the nucleus of hippocampal neurones after DHPG treatment was shown by immunofluorescence using a phosphospecific antibody. We suggest that LTD leads to SRF/Elk-1-regulated gene expression of immediate early transcription factors, which could in turn promote a second broader wave of gene expression.

Co-localization of midbrain projections, progestin receptors, and mating-induced fos in the hypothalamic ventromedial nucleus of the female rat

In female rats, sexual behavior requires the convergence of ovarian hormone signals, namely estradiol and progesterone, and sensory cues from the male on a motor output pathway. Estrogen and progestin receptors (ER and PR) are found in neurons in the hypothalamic ventromedial nucleus (VMH), a brain region necessary for lordosis, the stereotypic female copulatory posture. A subset of VMH neurons sends axonal projections to the periaqueductal gray (PAG) to initiate a motor output relay, and some of these projection neurons express PR. Previous studies showed that VMH neurons are activated during mating, based on the expression of the immediate early gene Fos. Many of the activated neurons expressed ER; however, it is not known if such activated neurons co-express PR. Fluorogold, a retrograde tracer, was injected into the PAG of ovariectomized rats to label neurons projecting from the VMH. Hormone-treated animals then were mated, and their brains were immunohistochemically stained for PR and Fos. Of the Fos-positive neurons, 33% were double-labeled for PR, 19% were double-labeled with Fluorogold, and 5% were triple-labeled for Fos, PR, and the retrograde tracer. The majority of triple-labeled neurons were found in the rostral, rather than caudal, portion of the VMH. These results show that PR-containing neurons are engaged during sexual behavior, which suggests that these neurons are the loci of hormonal-sensory convergence and hormonal-motor integration.

Specific plasticity responses to unilaterally decreased or increased hearing intensity in the adult cochlear nucleus and beyond

Variations of sensory activation in strength and pattern are known to affect structure and function of the mammalian brain. Whereas such malleability is readily granted to forebrain structures at early developmental stages, acceptance of experience-dependent structural plasticity has been slow for the adult brainstem. Over the past years we have identified consequences of cochlear ablation, noise trauma, or electrical intracochlear stimulation on neurons and circuitry of the auditory brainstem of the adult rat. We found that loss of sensory activation as well as a substitution for it entail specific molecular, ultrastructural, and morphological changes to central auditory neurons. Here, we make a first attempt to compare these different patterns of central remodeling. We tentatively suggest that after hearing loss or intracochlear stimulation responses of the central neural network in the adult brainstem suit the concept of functional adaptation.

In vivo investigation of genome activity and synaptic plasticity of neurons in snails during learning

The present report describes in vivo investigations of genome activity and its role in the mechanisms forming long-term synaptic plasticity in defensive behavior command neuron LPl1 during the acquisition of nociceptive sensitization by common snails. Transcription processes were recorded using SYTO 16, a specific fluorescent indicator of DNA activity, along with in vivo computer image analysis. Studies in control snails showed that application of nociceptive stimuli to the head led to biphasic changes in the bioelectrical responses of neurons to tactile and chemical stimulation--depression of responses in the short-term stage (during the 1 h after training) and their facilitation during the long-term stage of sensitization (more than 24 h). There were marked increases in fluorescence over the nucleus of the command neuron stained with SYTO 16 at 15-20 min from the start of training, this lasting 4-5 h. Acquisition of sensitization in the presence of the RNA synthesis inhibitor actinomycin D (20 microM) to the neuron led to the complete elimination of changes in fluorescence and synaptic facilitation in the responses of LPl1 to sensory stimulation in the long-term stage of sensitization but had no effect during the short-term stage of sensitization. Actinomycin D given 30 min after the end of acquisition of sensitization (1 h after the start) had no effect on the dynamics of fluorescence or synaptic facilitation. Thus, the acquisition of nociceptive sensitization is accompanied by a rapid (within 15-20 min) activation of the DNA of neuron LPl1 and subsequent (about 1 h) display of long-term synaptic facilitation. Induction of both processes was suppressed by the RNA synthesis inhibitor over a relatively short time period--1 h from the moment at which training started.

Induction of immediate early gene expression by high-frequency stimulation of the subthalamic nucleus in rats

Deep brain stimulation is associated with delayed improvement of parkinsonian symptoms, such as hypokinesia with subthalamic nucleus stimulation, or dystonia with globus pallidus internus stimulation. The latency observed is better explained by molecular alterations than immediate electrophysiological processes, and clinical improvement may involve adaptive gene expression. Here, we have studied immediate early gene expression as fast molecular response to subthalamic nucleus stimulation. Bipolar electrodes were implanted bilaterally into the subthalamic nucleus of anesthetized male Wistar rats. High-frequency stimulation (130 Hz or 80 Hz, 60 micros, 300 microA) or low-frequency stimulation (5 Hz, 60 micros, 300 microA) was performed with the right electrode for 15, 60, 120, and 240 min whereas the silent left electrode served as negative control. Brains were fixed by transcardial perfusion and frozen sections were stained with polyclonal antibodies directed against three immediate early gene-encoded proteins, c-Fos, c-Jun, and Krox-24 (NGFI-A, Egr-1, Zif268, Tis8, Zenk). After 120 and 240 h, c-Fos immunoreactivity was strongly upregulated in subthalamic nucleus neurons on the stimulated site. In contrast, no c-Fos immunoreactivity was detected on the non-stimulated site except for single positive cells located in close proximity to the electrode tracks. Furthermore, c-Fos immunoreactivity was induced in subthalamic nucleus projection areas, such as primary and secondary motor cortex, primary somatosensory and insular cortex, lateral and medial globus pallidus, suprageniculate thalamic nucleus, pontine nuclei, medial geniculate nucleus, and substantia nigra. Similarly, c-Jun and Krox-24 were induced at the site of stimulation and in projection areas following high-frequency subthalamic nucleus stimulation. Whereas high frequency stimulation with 80 Hz was similarly effective none of the three immediate early gene-encoded proteins was induced with low-frequency stimulation (5 Hz) for 4 h. This is in accordance with the therapeutic effects of deep brain stimulation which are only elicited with high frequency stimulation. Our data provide evidence that immediate early gene expression in the subthalamic nucleus is rapidly and substantially induced by high-frequency stimulation. The induction of immediate early genes in projection sites suggests ipsilateral transsynaptic modulation of neuronal activity.

Long-term potentiation and memory

One of the most significant challenges in neuroscience is to identify the cellular and molecular processes that underlie learning and memory formation. The past decade has seen remarkable progress in understanding changes that accompany certain forms of acquisition and recall, particularly those forms which require activation of afferent pathways in the hippocampus. This progress can be attributed to a number of factors including well-characterized animal models, well-defined probes for analysis of cell signaling events and changes in gene transcription, and technology which has allowed gene knockout and overexpression in cells and animals. Of the several animal models used in identifying the changes which accompany plasticity in synaptic connections, long-term potentiation (LTP) has received most attention, and although it is not yet clear whether the changes that underlie maintenance of LTP also underlie memory consolidation, significant advances have been made in understanding cell signaling events that contribute to this form of synaptic plasticity. In this review, emphasis is focused on analysis of changes that occur after learning, especially spatial learning, and LTP and the value of assessing these changes in parallel is discussed. The effect of different stressors on spatial learning/memory and LTP is emphasized, and the review concludes with a brief analysis of the contribution of studies, in which transgenic animals were used, to the literature on memory/learning and LTP.

Adenylyl cyclase-dependent form of chemical long-term potentiation triggers translational regulation at the elongation step

The persistent maintenance of long-term potentiation requires both messenger RNA and protein synthesis. While there is mounting evidence for an active role of protein synthesis in hippocampal long-term potentiation, the nature of mechanisms underlying its regulation has not yet been established. We used a previously described chemical long-term potentiation protocol [J Neurosci 19 (1999) 2500] to address the hypothesis that signaling mechanisms, involved in long-lasting long-term potentiation, directly regulate protein synthesis. Chemical long-term potentiation is an N-methyl-D-aspartate receptor-dependent form of plasticity, which relies on both synaptic activity, in the form of spontaneous bursting induced by high concentrations of K(+) and Ca(2+), and cyclic AMP/adenylyl cyclase signaling. We found that chemical long-term potentiation in CA1 of the mouse hippocampus lasts for at least 3 hours and requires both messenger RNA and protein synthesis. However, surprisingly de novo total protein synthesis was paradoxically decreased at 1 hour after long-term potentiation induction. Consistent with the decrease in total protein synthesis in potentiated CA1, phosphorylation of eukaryotic elongation factor 2 was increased and is likely responsible for inhibition of translation at the elongation step. Increased phosphorylation of eukaryotic elongation factor 2 was dependent on coincident cyclic AMP/adenylyl cyclase activation and synaptic activity and required N-methyl-D-aspartate receptor activation. Despite the inhibition in total protein synthesis, the level of the immediate early gene protein Arc (activity regulated cytoskeleton-associated protein) increased at 1 hour after chemical long-term potentiation induction. Taken together, the results suggest that regulation at the elongation step of protein synthesis contributes to persistent forms of long-term potentiation.

Expression of the specific type IV phosphodiesterase gene PDE4B3 during different phases of long-term potentiation in single hippocampal slices of rats in vitro

Hippocampal long-term potentiation (LTP), the most prominent cellular model for learning and memory formation, consists of phases: early-LTP (<4 h) and late-LTP (>4 h), with the latter dependent upon protein translation and transcription. To explore the molecular processes that might be specifically regulated during late-LTP, we have modified standard electrophysiological and molecular biological methods, which allowed the cloning of activated genes and their products from single hippocampal slices in vitro 8 h after LTP induction. From one such screen we identified a specific type IV phosphodiesterase gene, PDE4B3, the first cAMP-specific phosphodiesterase to be associated with LTP. Previous studies documented an integral role for the cAMP-PKA system in late-LTP and recently, inhibition of cAMP degradation facilitates LTP and ameliorates mnemonic deficits. We now report that PDE4B3 is modulated during LTP phases. Its activation is NMDA-receptor dependent and its transcription is transiently up-regulated 2 h after tetanization. Protein expression peaks 6 h after LTP induction and is rapidly down-regulated at 8 h, whereas cAMP levels decrease during LTP phases. Immunohistochemical studies identified that the majority of type IV phosphodiesterase protein staining is localized to the cell bodies and dendrites of neurones in hippocampal CA1.

Formation of AP-1 transcription factors during learning in Helix

Gel shift assays were used to study the formation of transcription factors of the AP-1 family in the CNS of Helix during the acquisition of a conditioned defensive reflex based on food aversion. Increases in the DNA-binding activity of AP-1 factors were seen 1-3 h after training. Modeling of "learning" in an in situ system (incubation of the CNS in the presence of serotonin and the Ca2+ ionophore A23187 or the protein kinase C activator phorbol ester (TPA)) also increased the DNA-binding activity of this transcription complex. The DNA-binding activity induced by serotonin acting alone was significantly less than that in controls, while that in the presence of A23187 and TPA was the same as or less than that in controls. The calcium/calmodulin-dependent protein kinase inhibitor KN62 produced significant suppression of the effects of simultaneous exposure to serotonin and calcium on the activation of transcription factors of the AP-1, while greater suppression was obtained with the mitogen-activated protein kinase (ERK) inhibitor PD98059. Cooperative induction of the activation of AP-1 transcription factors in the CNS of the common snail by the serotonin-induced and calcium-dependent regulatory systems may be a mechanism underlying the formation of conditioned defensive reflexes in these animals. Lesions of the formation of transcription factors of the AP-1 family in animals unable to learn defensive forms of behavioral plasticity may be explained in terms of the presence of inhibitory forms of transcription factors interacting with the SRE element.

Spatio-temporal heterogeneity and cell-specificity of long-term potentiation-induced mRNA expression in the dentate gyrus in vivo

Gene expression in neurones can vary in response to neuronal activation. In this study, to analyse the spatio-temporal dynamics of the transcriptional response of three genes following the induction of long-term potentiation within the entire dentate gyrus in vivo, two new complementary approaches based on in situ hybridisation were developed: three-dimensional reconstruction of the pattern of mRNA expression within the entire dentate gyrus; and radioactive co-detection of two mRNA species allowing quantification of two different mRNAs in the same brain section. Zif268, Homer and syntaxin 1B genes were studied, and their regulated expression was examined three times after the induction of long-term potentiation. Constitutive expression of each gene under control conditions was homogeneous, but the spatial distribution of mRNA was heterogeneous along the rostro-caudal axis of the dentate gyrus following the induction of long-term potentiation, and different for each gene. In addition, the intensity of each gene-specific pattern of expression varied over time following the induction of long-term potentiation. Our results reveal that long-term potentiation differentially modulates the expression of mRNA species in cells of the dentate gyrus depending on their position along the rostro-caudal axis, on the gene and on time. We suggest that there are several molecular mechanisms of long-term potentiation, differing from one cluster of cells of the dentate gyrus to another, or that the different signaling pathways involved in long-term potentiation are used with varying efficiencies by different cells.

Long-term potentiation of thalamocortical transmission in the adult visual cortex in vivo

It has been suggested that NMDA receptor-dependent synaptic strengthening, like that observed after long-term potentiation (LTP), is a mechanism by which experience modifies responses in the neocortex. We report here that patterned (theta burst) stimulation of the dorsal lateral geniculate nucleus reliably induces LTP of field potentials (FPs) evoked in primary visual cortex (Oc1) of adult rats in vivo. The response enhancement is saturable, long-lasting, and dependent on NMDA receptor activation. To determine the laminar locus of these changes, current source density (CSD) analysis was performed on FP profiles obtained before and after LTP induction. LTP was accompanied by an enhancement of synaptic current sinks located in thalamorecipient (layer IV and deep layer III) and supragranular (layers II/III) cell layers. We also examined immunocytochemical labeling for the immediate early gene zif-268 1 hr after induction of LTP. In concert with the laminar changes observed in CSD analyses, we observed a significant increase in the number of zif-268-immunopositive neurons in layers II-IV that occurred over a wide extent of Oc1. Last, we investigated the functional consequences of LTP induction by monitoring changes in visually evoked potentials. After LTP, we observed that the cortical response to a full-field flash was significantly enhanced and that responses to grating stimuli were increased across a range of spatial frequencies. These findings are consistent with growing evidence that primary sensory cortex remains plastic into adulthood, and they show that the mechanisms of LTP can contribute to this plasticity.

Protein synthesis in entorhinal cortex and long-term potentiation in dentate gyrus

Despite the concentration of effort in recent years, the mechanisms underlying the expression of long-term potentiation (LTP) in the hippocampus remain elusive, but amidst the uncertainty and sometimes controversy, one consistent finding is emerging; this is that late-phase LTP requires synthesis of proteins. This hypothesis was first proposed by a number of groups who reported that the more persistent components of LTP were blocked by protein synthesis inhibitors, and was supported by a significant literature which indicated that morphological changes accompanied LTP. Recent evidence indicated that the increase in protein synthesis may be cAMP-dependent and that subsequent activation of the transcription factor, CREB, represented one step in the cascade of events leading to protein synthesis. Whether protein synthesis occurs in presynaptic or postsynaptic neurons, or both, is still a subject of debate. Here we present evidence which suggests that LTP in perforant path-granule cell synapses is accompanied by protein synthesis, specifically synthesis of synaptic vesicle proteins, in the entorhinal cortex. We also show that protein synthesis is decreased in the entorhinal cortex of aged rats and a strain of rat which is genetically hypertensive, both of which exhibited impaired LTP. We propose that that the observed increase in protein synthesis in the entorhinal cortex, which accompanied LTP in the dentate gyrus, contributes to the reported changes in morphology in the presynaptic terminal.

The genomic action potential

Neurons compute in part by integrating, on a time scale of milliseconds, many synaptic inputs and generating a digital output-the "action potential" of classic electrophysiology. Recent discoveries indicate that neurons also perform a second, much slower, integration operating on a time scale of minutes or even hours. The output of this slower integration involves a pulse of gene expression which may be likened to the electrophysiological action potential. Its function, however, is not directed toward immediate transmission of a synaptic signal but rather toward the experience-dependent modification of the underlying synaptic circuitry. Commonly termed the "immediate early gene" (IEG) response, this phenomenon is often assumed to be a necessary component of a linear, deterministic cascade of memory consolidation. Critical review of the large literature describing the phenomenon, however, leads to an alternative model of IEG function in the brain. In this alternative, IEG activation is not directed at the consolidation of memories of a specific inducing event; instead, it sets the overall gain or efficiency of memory formation and directs it to circuits engaged by behaviorally significant contexts. The net result is a sharpening of the selectivity of memory formation, a recruitment of temporally correlated associations, and an ultimate enhancement of long-term memory retrieval.

Hippocampal expression of the orphan nuclear receptor gene hzf-3/nurr1 during spatial discrimination learning

The immediate-early gene hzf-3, also known as nurr1, is a member of the inducible orphan nuclear receptor family and is one candidate in the search for genes associated with learning and memory processes. Here we report that acquisition of a spatial food search task is accompanied by elevated levels of hzf-3 mRNA in the hippocampus. Adult male Long-Evans rats were handled, food-restricted, and allowed to habituate to the maze prior to training. During acquisition, rats were given one training session per day for 5 days. Each training session consisted of five trials in which animals searched the maze for food located in 4 of 16 holes in the floor of the maze. Training resulted in spatial acquisition of the task. Northern blot analysis showed significant increases in hippocampal hzf-3 mRNA 3 h after training in the maze. Next, brains were obtained from Naive, Habituated, Day 1, Day 3, and Day 5 animals and processed for in situ hybridization. The results showed significant increases of hzf-3 mRNA in CA1 and CA3 subregions of the dorsal hippocampus during acquisition of the task. We conclude that expression of the hzf-3 gene in the brain is associated with long-term spatial memory processes. The present results are the first to implicate an orphan nuclear receptor in long-term information storage in the hippocampus.

Identification and cataloging of genes induced by long-lasting long-term potentiation in awake rats

Maintenance of long-term potentiation (LTP) requires de novo gene expression. Here we report the direct isolation, using PCR-differential display, of genes whose expression level was altered after induction of long-lasting LTP in the hippocampus of freely moving awake rats. Differential display using 480 primer combinations revealed 17 cDNA bands that showed a reproducible change in expression level. These cDNAs represented at least 10 different genes (termed RM1-10), all of which showed up-regulation at 75 min after LTP induction and a return to basal expression levels within 24 h. Three of these genes were known only from expressed sequence tags (RM1-3), two were known genes whose up-regulation by LTP has not been described (GADD153/CHOP and ler5), and five were known genes whose up-regulation by LTP has already been reported (MAPK phosphatase, NGFI-A/zif268, vesl-1S/homer-1a, Ag2, and krox-20). We characterized the expression profiles of genes in the two former categories with respect to NMDA receptor dependency, tissue specificity, and developmental regulation using northern blotting and semiquantitative RT-PCR. The up-regulation of all five of these genes was NMDA receptor-dependent and correlated with the persistence of LTP, suggesting that these genes may play functional roles in prolonged LTP maintenance.

Sequential increase in Egr-1 and AP-1 DNA binding activity in the dentate gyrus following the induction of long-term potentiation

Establishment of long-term potentiation (LTP) at perforant path synapses is highly correlated with increased expression of Egr and AP-1 transcription factors in rat dentate gyrus granule cells. We have investigated whether increased transcription factor levels are reflected in increased transcription factor activity by assessing Egr and AP-1 DNA binding activity using gel shift assays. LTP produced an increase in binding to the Egr element, which was NMDA receptor-dependent and correlated closely with our previously reported increase in Egr-1 (zif/268) protein levels. Supershift analysis confirmed involvement of Egr-1, but not Egr-2 in the DNA binding activity. AP-1 DNA binding was also rapidly elevated in parallel with protein levels, however, the peak increase in activity was delayed until 4 h, a time point when we have previously shown that only jun-D protein was elevated. These data indicate that binding of Egr-1 and AP-1 to their response elements is increased in two phases. This may result in activation of distinct banks of target genes which contribute to the establishment of persistent LTP.

Possible molecular-cellular mechanisms of the regulation of gene expression during learning

This study is an analysis of the regulatory mechanisms of plasticity. The first part provides a short review of the role of DNA-binding transcription factors in possible regulatory pathways and their activity in the mechanisms of plasticity. Our own data obtained in studies of the molecular mechanisms underlying the formation of conditioned defensive reflexes in Helix are then presented. These studies show that formation of defensive types of plasticity in Helix is accompanied by serotonin-induced translocation of a protein with Rf 0.58 and increases in G-protein activity, protein kinase A activity, and expression of the c-fos gene. Transcription factors CRE and AP-1 probably have roles in the learning process. Gel shift assays demonstrated the existence of transcription factors of the CRE and AP-1 families in adult snails. In juvenile snails, which were unable to form defensive types of plasticity, the serotonin protein with Rf 0.58 (the learning "marker") was absent from and was not induced in the CNS. Gel shift assay results also showed that transcription factors of the AP-1 family were not present and were not induced by serotonin or the protein kinase A activator forskolin, though these snails had significant levels of CRE transcription factors. Serotonin and forskolin increased the DNA-binding activity of CRE in juvenile Helix. The lack of activity of transcription factors of the AP-1 family in juvenile snails may explain their inability to development sensitization and conditioned defensive reflexes.

Roles of nuclear factor kappaB in neuronal survival and plasticity

The transcription factor nuclear factor kappaB (NF-kappaB) is moving to the forefront of the fields of apoptosis and neuronal plasticity because of recent findings showing that activation of NF-kappaB prevents neuronal apoptosis in various cell culture and in vivo models and because NF-kappaB is activated in association with synaptic plasticity. Activation of NF-kappaB was first shown to mediate antiapoptotic actions of tumor necrosis factor in cultured neurons and was subsequently shown to prevent death of various nonneuronal cells. NF-kappaB is activated by several cytokines and neurotrophic factors and in response to various cell stressors. Oxidative stress and elevation of intracellular calcium levels are particularly important inducers of NF-kappaB activation. Activation of NF-kappaB can interrupt apoptotic biochemical cascades at relatively early steps, before mitochondrial dysfunction and oxyradical production. Gene targets for NF-kappaB that may mediate its antiapoptotic actions include the antioxidant enzyme manganese superoxide dismutase, members of the inhibitor of apoptosis family of proteins, and the calcium-binding protein calbindin D28k. NF-kappaB is activated by synaptic activity and may play important roles in the process of learning and memory. The available data identify NF-kappaB as an important regulator of evolutionarily conserved biochemical and molecular cascades designed to prevent cell death and promote neuronal plasticity. Because NF-kappaB may play roles in a range of neurological disorders that involve neuronal degeneration and/or perturbed synaptic function, pharmacological and genetic manipulations of NF-kappaB signaling are being developed that may prove valuable in treating disorders ranging from Alzheimer's disease to schizophrenia.

Expression of Fos and Jun Proteins Following Passive Avoidance Training in the Day-Old Chick

It has been shown previously that the immediate-early genes, c-fos and c-jun mRNA are induced in the 1-day-old chick forebrain after one-trial passive avoidance training in which chicks learn to avoid pecking at a bitter-tasting bead. Here, we have studied the expression of their proteins using antibodies to Fos and Jun. Western blotting disclosed two immunoreactive bands for the anti-Fos antibody (47 and 54 kD) and two immunoreactive bands for the anti-Jun antibody (39 and 54 kD). Two hours post-training there was an increase in the number of Fos-positive stained nuclei in right intermediate medial hyperstriatum ventrale (IMHV) (P < 0.01), left IMHV (P < 0.05), right lobus parolfactorius (LPO) (P < 0.025) and left LPO (P < 0.05) of birds trained on the bitter bead compared with controls that had pecked a water-coated bead. Staining for Jun protein was significantly greater in the right LPO of trained chicks (P < 0.01). Other forebrain regions showed no increase over quiet control levels. The findings are discussed in the context of the cascade of events involved in passive avoidance memory consolidation in the day-old chick.

Activity and injury-dependent expression of inducible transcription factors, growth factors and apoptosis-related genes within the central nervous system

This review primarily discusses work that has been performed in our laboratories and that of our direct collaborators and therefore does not represent an exhaustive review of the current literature. Our aim is to further discuss the role that gene expression plays in neuronal plasticity and pathology. In the first part of this review we examine activity-dependent changes in the expression of inducible transcription factors (ITFs) and neurotrophins with long-term potentiation (LTP) and kindling. This work has identified particular ITFs (Krox-20 and Krox-24) and neurotrophin systems (particularly the brain-derived neurotrophic factor (BDNF)/tyrosine receptor kinase-B, Trk-B system) that may be involved in stabilizing long-lasting LTP (i.e. LTP3). We also show that changes in the expression of other ITFs (Fos, Jun-D and Krox-20) and the BDNF/trkB neurotrophin system may play a central role in the development of hippocampal kindling, an animal model of human temporal lobe epilepsy. In the next part of this review we examine changes in gene expression after neuronal injuries (ischemia, prolonged seizure activity and focal brain injury) and after nerve transection (axotomy). We identify apoptosis-related genes (p53, c-Jun, Bax) whose delayed expression selectively increases in degenerating neurons, further suggesting that some forms of neuronal death may involve apoptosis. Moreover, since overexpression of the tumour-suppressor gene p53 induces apoptosis in a wide variety of dividing cell types we speculate that it may perform the same function in post-mitotic neurons following brain injuries. Additionally, we show that neuronal injury is associated with rapid, transient, activity-dependent expression of neurotrophins (BDNF and activinA) in neurons, contrasting with a delayed and more persistent injury-induced expression of certain growth factors (IGF-1 and TGFbeta) in glia. In this section we also describe results linking ITFs and neurotrophic factor expression. Firstly, we show that while BDNF and trkB are induced as immediate-early genes following injury, the injury-induced expression of activinA and trkC may be regulated by ITFs. We also discuss whether loss of retrograde transport of neurotrophic factors such as nerve growth factor following nerve transection triggers the selective and prolonged expression of c-Jun in axotomized neurons and whether c-Jun is responsible for regeneration or degeneration of these axotomized neurons. In the last section we further examine the role that gene expression may play in memory formation, epileptogenesis and neuronal degeneration, lastly speculating whether the expression of various growth factors after brain injury represents an endogenous neuroprotective response of the brain to injury. Here we discuss our results which show that pharmacological enhancement of this response with exogenous application of IGF-1 or TGF-beta reduces neuronal loss after brain injury.

Unique regulation of immediate early gene and tyrosine hydroxylase expression in the odor-deprived mouse olfactory bulb

Tyrosine hydroxylase (TH), expressed in a population of periglomerular neurons intrinsic to the olfactory bulb, displays dramatic down-regulation in response to odor deprivation. To begin to elucidate the importance of immediate early genes (IEG) in TH gene regulation, the present study examined expression of IEGs in the olfactory bulb in response to odor deprivation. In addition, the composition of TH AP-1 and CRE binding complexes was investigated in control and odor-deprived mice. Immunocytochemical studies showed that c-Fos, Fos-B, Jun-D, CRE-binding protein (CREB), and phosphorylated CREB (pCREB) are colocalized with TH in the dopaminergic periglomerular neurons. Unilateral naris closure resulted in down-regulation of c-Fos and Fos-B, but not Jun-D, CREB, or pCREB, in the glomerular layer of the ipsilateral olfactory bulb. Gel shift assays demonstrated a significant decrease (32%) in TH AP-1, but not CRE, binding activity in the odor-deprived bulb. Fos-B was found to be the exclusive member of the Fos family present in the TH AP-1 complex. CREB, CRE modulator protein (CREM), Fos-B, and Jun-D, but not c-Fos, all contributed to the CRE DNA-protein complex. These results indicated that Fos-B, acting through both AP-1 and CRE motifs, may be implicated in the regulation of TH expression in the olfactory bulb dopaminergic neurons.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

Occurrence of a transcription factor, cAMP response element-binding protein (CREB), in the postsynaptic sites of the brain

The postsynaptic density (PSD) fraction prepared from the rat forebrain contained a transcription factor, cAMP response element-binding protein (CREB). The occurrence of CREB in the PSD was confirmed by immunoelectron microscopic examination. CREB in the PSD fraction was phosphorylated both by protein kinase A and Ca2+/calmodulin-dependent protein kinase II (CaMKII) endogenous to the fraction, and dissociated from the PSD after phosphorylation, especially under CaMKII-activated conditions. The fraction containing CREB that was released from PSD after phosphorylation possessed cAMP response element (CRE)-binding activity. Thus, PSD anchors functionally active CREB. These results suggest that CREB anchored to the PSD is liberated by phosphorylation upon specific synaptic stimulation, translocates into the nucleus, and then triggers synaptic activity-dependent changes in gene expression.

Effects of prolactin on expression of the mRNAs encoding the immediate early genes zif/268 (NGF1-A), nur/77 (NGF1-B), c-fos and c-jun in the hypothalamus

Prolactin (PRL) exerts a short-loop negative feedback effect on hypothalamic neurons which control its secretion from the anterior pituitary gland. The purpose of this study was to identify the location of hypothalamic neurons which respond to acute PRL exposure. Increasing evidence indicates that excitation of neurons often results in the rapid transcription of immediate early genes (IEGs). In the present study, quantitative in situ hybridization histochemistry (ISHH) was used to visualize the induction of mRNAs for four different IEGs: zif/268 (NGF1-A), nur/77 (NGF1-B), c-fos and c-jun. Three groups of male rats were compared: unmanipulated controls, rats injected s.c. with 2.4 mg ovine PRL (oPRL) suspended in polyvinylpyrrolidone (PVP), and PVP-injected controls. Animals were decapitated 0, 0.5, 1, 2, 3 or 4 h following injection. In all rats, the four probes labeled cells within the cortex, particularly the cingulate and piriform cortices, the hippocampus and the striatum. In the arcuate nucleus, there was a modest increase in the average number of cells/animal which expressed zif/268 mRNA following the injection of PVP and oPRL at all times studied. The average area of grains/cell representing zif/268 message also increased following the injection stimulus. The number of neurons expressing nur/77 mRNA was greater in PRL-treated rats compared with PVP-treated controls 0.5 and 1 h following injection. Nur/77-labeled neurons were co-extensive with the tuberoinfundibular dopaminergic (TIDA) neurons. The data suggest that cells located within the arcuate nucleus are involved in mediating PRL autofeedback on the brain.

Biphasic changes in the levels of N-methyl-D-aspartate receptor-2 subunits correlate with the induction and persistence of long-term potentiation

N-Methyl-D-aspartate glutamate receptors (NMDAR) form ion channels made up of polypeptides from two classes of subunits; NR1 is obligatory for function whereas members of the NR2 class regulate the properties of the channel. Long-term potentiation (LTP) of synaptic transmission is an event largely dependent on NMDAR activation, and is studied as the primary cellular model of memory in the mammalian brain. While there has been a focus on non-NMDARs in mediating the expression of LTP, we report here biochemical evidence for plasticity of the NMDAR that is associated with LTP persistence in awake animals. Following the establishment of LTP in perforant path synapses of the dentate gyrus, we observed a rise in NR2B protein levels 48 h post-tetanus which was dependent upon activation of NMDARs during the tetanization, and which strongly correlated with the degree of LTP measured at this time-point. We also observed a transient increase in both NR2B and NR2A protein levels 20 min post-tetanus that returned to control levels by 4 h. These early increases were not observed in anaesthetized animals which do not sustain persistent LTP. Our data demonstrate a marked plasticity of NMDAR subunit expression, which may affect LTP persistence, as well as the subsequent ability to induce LTP at previously activated synapses.

Hippocampal plasticity involves extensive gene induction and multiple cellular mechanisms

Long-term plasticity of the central nervous system (CNS) involves induction of a set of genes whose identity is incompletely characterized. To identify candidate plasticity-related genes (CPGs), we conducted an exhaustive screen for genes that undergo induction or downregulation in the hippocampus dentate gyrus (DG) following animal treatment with the potent glutamate analog, kainate. The screen yielded 362 upregulated CPGs and 41 downregulated transcripts (dCPGs). Of these, 66 CPGs and 5 dCPGs are known genes that encode for a variety of signal transduction proteins, transcription factors, and structural proteins. Seven novel CPGs predict the following putative functions: cpg2--a dystrophin-like cytoskeletal protein; cpg4--a heat-shock protein: cpg16--a protein kinase; cpg20--a transcription factor; cpg21--a dual-specificity MAP-kinase phosphatase; and cpg30 and cpg38--two new seven-transmembrane domain receptors. Experiments performed in vitro and with cultured hippocampal cells confirmed the ability of the cpg-21 product to inactivate the MAP-kinase. To test relevance to neural plasticity, 66 CPGs were tested for induction by stimuli producing long-term potentiation (LTP). Approximately one-fourth of the genes examined were upregulated by LTP. These results indicate that an extensive genetic response is induced in mammalian brain after glutamate receptor activation, and imply that a significant proportion of this activity is coinduced by LTP. Based on the identified CPGs, it is conceivable that multiple cellular mechanisms underlie long-term plasticity of the nervous system.

Long-term effects of axotomy on excitability and growth of isolated Aplysia sensory neurons in cell culture: potential role of cAMP

Crushing nerves, which contain the axons of central sensory neurons, in Aplysia causes the neurons to become hyperexcitable and to sprout new processes. Previous experiments that examined the effects of axonal injury on Aplysia sensory neurons have been performed in the intact animal or in the semi-intact CNS of Aplysia. It therefore has been unclear to what extent the long-term neuronal consequences of injury are due to intrinsic or extrinsic cellular signals. To determine whether injury-induced changes in Aplysia sensory neurons are due to intrinsic or extrinsic signals, we have developed an in vitro model of axonal injury. Isolated central sensory neurons grown for 2 days in cell culture were axotomized. Approximately 24 h after axotomy, sensory neurons exhibited a greater excitability-reflected, in part, as a significant reduction in spike accommodation-and greater neuritic outgrowth than did control (unaxotomized) neurons. Rp diastereoisomer of the cyclic adenosine 3',5'-monophosphorothiate (Rp-cAMPS), an inhibitor of protein kinase A, blocked both the reduction in accommodation and increased neuritic outgrowth induced by axotomy. Rp-cAMPS also blocked similar, albeit smaller, alterations observed in control sensory neurons during the 24-h period of our experiments. These results indicate that axonal injury elevates cAMP levels within Aplysia sensory neurons, and that this elevation is directly responsible, in part, for the previously described long-term electrophysiological and morphological changes induced in Aplysia sensory neurons by nerve crush. In addition, the results indicate that control sensory neurons in culture are also undergoing injury-related electrophysiological and structural changes, probably due to cellular processes triggered when the neurons are axotomized during cell culturing. Finally, the results provide support for the idea that the cellular processes activated within Aplysia sensory neurons by injury, and those activated during long-term behavioral sensitization, overlap significantly.

A molecular biological approach to synaptic plasticity and learning

Until the more recent advances made in molecular biology, attempts to link synaptic plasticity and learning have focused on using LTP as a marker of learning-induced synaptic plasticity, where one has expected to observe the same magnitude of change in synaptic strength as that observed with artificial stimulation. To a large extent this approach has been frustrated by the fact that it is generally assumed that the representation of the memory traces is distributed throughout widespread networks of cells. By implication it is more likely that one would observe small distributed changes within a network; a formidable task to measure. In this review we describe how the advances in molecular biology give us both the tools to investigate the mechanisms of synaptic plasticity and to apply these to investigations of the underlying mechanisms in learning and the formation of memories that have until now remained out of our grasp.