Conditioned eyeblink response is impaired in mutant mice lacking NMDA receptor subunit NR2A

The blink reflex and its modulation - Part 1: Physiological mechanisms

The blink reflex (BR) is a protective eye-closure reflex mediated by brainstem circuits. The BR is usually evoked by electrical supraorbital nerve stimulation but can be elicited by a variety of sensory modalities. It has a long history in clinical neurophysiology practice. Less is known, however, about the many ways to modulate the BR. Various neurophysiological techniques can be applied to examine different aspects of afferent and efferent BR modulation. In this line, classical conditioning, prepulse and paired-pulse stimulation, and BR elicitation by self-stimulation may serve to investigate various aspects of brainstem connectivity. The BR may be used as a tool to quantify top-down modulation based on implicit assessment of the value of blinking in a given situation, e.g., depending on changes in stimulus location and probability of occurrence. Understanding the role of non-nociceptive and nociceptive fibers in eliciting a BR is important to get insight into the underlying neural circuitry. Finally, the use of BRs and other brainstem reflexes under general anesthesia may help to advance our knowledge of the brainstem in areas not amenable in awake intact humans. This review summarizes talks held by the Brainstem Special Interest Group of the International Federation of Clinical Neurophysiology at the International Congress of Clinical Neurophysiology 2022 in Geneva, Switzerland, and provides a state-of-the-art overview of the physiology of BR modulation. Understanding the principles of BR modulation is fundamental for a valid and thoughtful clinical application (reviewed in part 2) (Gunduz et al., submitted).

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

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

Real-Time Simulation of Passage-of-Time Encoding in Cerebellum Using a Scalable FPGA-Based System

The cerebellum plays a critical role for sensorimotor control and learning. However, dysmetria or delays in movements' onsets consequent to damages in cerebellum cannot be cured completely at the moment. Neuroprosthesis is an emerging technology that can potentially substitute such motor control module in the brain. A pre-requisite for this to become practical is the capability to simulate the cerebellum model in real-time, with low timing distortion for proper interfacing with the biological system. In this paper, we present a frame-based network-on-chip (NoC) hardware architecture for implementing a bio-realistic cerebellum model with  ∼ 100 000 neurons, which has been used for studying timing control or passage-of-time (POT) encoding mediated by the cerebellum. The simulation results verify that our implementation reproduces the POT representation by the cerebellum properly. Furthermore, our field-programmable gate array (FPGA)-based system demonstrates excellent computational speed that it can complete 1sec real world activities within 25.6 ms. It is also highly scalable such that it can maintain approximately the same computational speed even if the neuron number increases by one order of magnitude. Our design is shown to outperform three alternative approaches previously used for implementing spiking neural network model. Finally, we show a hardware electronic setup and illustrate how the silicon cerebellum can be adapted as a potential neuroprosthetic platform for future biological or clinical application.

Burst-Dependent Bidirectional Plasticity in the Cerebellum Is Driven by Presynaptic NMDA Receptors

Numerous studies have shown that cerebellar function is related to the plasticity at the synapses between parallel fibers and Purkinje cells. How specific input patterns determine plasticity outcomes, as well as the biophysics underlying plasticity of these synapses, remain unclear. Here, we characterize the patterns of activity that lead to postsynaptically expressed LTP using both in vivo and in vitro experiments. Similar to the requirements of LTD, we find that high-frequency bursts are necessary to trigger LTP and that this burst-dependent plasticity depends on presynaptic NMDA receptors and nitric oxide (NO) signaling. We provide direct evidence for calcium entry through presynaptic NMDA receptors in a subpopulation of parallel fiber varicosities. Finally, we develop and experimentally verify a mechanistic plasticity model based on NO and calcium signaling. The model reproduces plasticity outcomes from data and predicts the effect of arbitrary patterns of synaptic inputs on Purkinje cells, thereby providing a unified description of plasticity.

The role of GluN2A and GluN2B NMDA receptor subunits in AgRP and POMC neurons on body weight and glucose homeostasis

OBJECTIVE

Hypothalamic agouti-related peptide (AgRP) and pro-opiomelanocortin (POMC) expressing neurons play critical roles in control of energy balance. Glutamatergic input via n-methyl-d-aspartate receptors (NMDARs) is pivotal for regulation of neuronal activity and is required in AgRP neurons for normal body weight homeostasis. NMDARs typically consist of the obligatory GluN1 subunit and different GluN2 subunits, the latter exerting crucial differential effects on channel activity and neuronal function. Currently, the role of specific GluN2 subunits in AgRP and POMC neurons on whole body energy and glucose balance is unknown.

METHODS

We used the cre-lox system to genetically delete GluN2A or GluN2B only from AgRP or POMC neurons in mice. Mice were then subjected to metabolic analyses and assessment of AgRP and POMC neuronal function through morphological studies.

RESULTS

We show that loss of GluN2B from AgRP neurons reduces body weight, fat mass, and food intake, whereas GluN2B in POMC neurons is not required for normal energy balance control. GluN2A subunits in either AgRP or POMC neurons are not required for regulation of body weight. Deletion of GluN2B reduces the number of AgRP neurons and decreases their dendritic length. In addition, loss of GluN2B in AgRP neurons of the morbidly obese and severely diabetic leptin-deficient Lep (ob/ob) mice does not affect body weight and food intake but, remarkably, leads to full correction of hyperglycemia. Lep (ob/ob) mice lacking GluN2B in AgRP neurons are also more sensitive to leptin's anti-obesity actions.

CONCLUSIONS

GluN2B-containing NMDA receptors in AgRP neurons play a critical role in central control of body weight homeostasis and blood glucose balance via mechanisms that likely involve regulation of AgRP neuronal survival and structure, and modulation of hypothalamic leptin action.

Loss of GluN2A-containing NMDA receptors impairs extra-dimensional set-shifting

Glutamate neurotransmission via the N-methyl-D-aspartate receptor (NMDAR) is thought to mediate the synaptic plasticity underlying learning and memory formation. There is increasing evidence that deficits in NMDAR function are involved in the pathophysiology of cognitive dysfunction seen in neuropsychiatric disorders and addiction. NMDAR subunits confer different physiological properties to the receptor, interact with distinct intracellular postsynaptic scaffolding and signaling molecules, and are differentially expressed during development. Despite these known differences, the relative contribution of individual subunit composition to synaptic plasticity and learning is not fully elucidated. We have previously shown that constitutive deletion of GluN2A subunit in the mouse impairs discrimination and re-learning phase of reversal when exemplars are complex picture stimuli, but spares acquisition and extinction of non-discriminative visually cued instrumental response. To investigate the role of GluN2A containing NMDARs in executive control, we tested GluN2A knockout (GluN2A(KO) ), heterozygous (GluN2A(HET) ) and wild-type (WT) littermates on an attentional set-shifting task using species-specific stimulus dimensions. To further explore the nature of deficits in this model, mice were tested on a visual discrimination reversal paradigm using simplified rotational stimuli. GluN2A(KO) were not impaired on discrimination or reversal problems when tactile or olfactory stimuli were used, or when visual stimuli were sufficiently easy to discriminate. GluN2A(KO) showed a specific and significant impairment in ventromedial prefrontal cortex-mediated set-shifting. Together these results support a role for GluN2A containing NMDAR in modulating executive control that can be masked by overlapping deficits in attentional processes during high task demands.

Role of motor cortex NMDA receptors in learning-dependent synaptic plasticity of behaving mice

The primary motor cortex has an important role in the precise execution of learned motor responses. During motor learning, synaptic efficacy between sensory and primary motor cortical neurons is enhanced, possibly involving long-term potentiation and N-methyl-D-aspartate (NMDA)-specific glutamate receptor function. To investigate whether NMDA receptor in the primary motor cortex can act as a coincidence detector for activity-dependent changes in synaptic strength and associative learning, here we generate mice with deletion of the Grin1 gene, encoding the essential NMDA receptor subunit 1 (GluN1), specifically in the primary motor cortex. The loss of NMDA receptor function impairs primary motor cortex long-term potentiation in vivo. Importantly, it impairs the synaptic efficacy between the primary somatosensory and primary motor cortices and significantly reduces classically conditioned eyeblink responses. Furthermore, compared with wild-type littermates, mice lacking primary motor cortex show slower learning in Skinner-box tasks. Thus, primary motor cortex NMDA receptors are necessary for activity-dependent synaptic strengthening and associative learning.

Motor cortex NMDA receptors have a key role in the acquisition of associative memories. Hasan et al. generate mice lacking NMDA receptor activity in the motor cortex and find that this impairs LTP, strengthening of synapses between somatosensory and motor cortices, and associative learning.

Age-dependent impairment of eyeblink conditioning in prion protein-deficient mice

Mice lacking the prion protein (PrP(C)) gene (Prnp), Ngsk Prnp (0/0) mice, show late-onset cerebellar Purkinje cell (PC) degeneration because of ectopic overexpression of PrP(C)-like protein (PrPLP/Dpl). Because PrP(C) is highly expressed in cerebellar neurons (including PCs and granule cells), it may be involved in cerebellar synaptic function and cerebellar cognitive function. However, no studies have been conducted to investigate the possible involvement of PrP(C) and/or PrPLP/Dpl in cerebellum-dependent discrete motor learning. Therefore, the present cross-sectional study was designed to examine cerebellum-dependent delay eyeblink conditioning in Ngsk Prnp (0/0) mice in adulthood (16, 40, and 60 weeks of age). The aims of the present study were two-fold: (1) to examine the role of PrP(C) and/or PrPLP/Dpl in cerebellum-dependent motor learning and (2) to confirm the age-related deterioration of eyeblink conditioning in Ngsk Prnp (0/0) mice as an animal model of progressive cerebellar degeneration. Ngsk Prnp (0/0) mice aged 16 weeks exhibited intact acquisition of conditioned eyeblink responses (CRs), although the CR timing was altered. The same result was observed in another line of PrP(c)-deficient mice, ZrchI PrnP (0/0) mice. However, at 40 weeks of age, CR incidence impairment was observed in Ngsk Prnp (0/0) mice. Furthermore, Ngsk Prnp (0/0) mice aged 60 weeks showed more significantly impaired CR acquisition than Ngsk Prnp (0/0) mice aged 40 weeks, indicating the temporal correlation between cerebellar PC degeneration and motor learning deficits. Our findings indicate the importance of the cerebellar cortex in delay eyeblink conditioning and suggest an important physiological role of prion protein in cerebellar motor learning.

Evolution of GluN2A/B cytoplasmic domains diversified vertebrate synaptic plasticity and behavior

Two genome duplications early in the vertebrate lineage expanded gene families, including GluN2 subunits of the NMDA receptor. Diversification between the four mammalian GluN2 proteins occurred primarily at their intracellular C-terminal domains (CTDs). To identify shared ancestral functions and diversified subunit-specific functions, we exchanged the exons encoding the GluN2A (also known as Grin2a) and GluN2B (also known as Grin2b) CTDs in two knock-in mice and analyzed the mice's biochemistry, synaptic physiology, and multiple learned and innate behaviors. The eight behaviors were genetically separated into four groups, including one group comprising three types of learning linked to conserved GluN2A/B regions. In contrast, the remaining five behaviors exhibited subunit-specific regulation. GluN2A/B CTD diversification conferred differential binding to cytoplasmic MAGUK proteins and differential forms of long-term potentiation. These data indicate that vertebrate behavior and synaptic signaling acquired increased complexity from the duplication and diversification of ancestral GluN2 genes.

IQGAP1 regulates NR2A signaling, spine density, and cognitive processes

General or brain-region-specific decreases in spine number or morphology accompany major neuropsychiatric disorders. It is unclear, however, whether changes in spine density are specific for an individual mental process or disorder and, if so, which molecules confer such specificity. Here we identify the scaffolding protein IQGAP1 as a key regulator of dendritic spine number with a specific role in cognitive but not emotional or motivational processes. We show that IQGAP1 is an important component of NMDAR multiprotein complexes and functionally interacts with the NR2A subunits and the extracellular signal-regulated kinase 1 (ERK1) and ERK2 signaling pathway. Mice lacking the IQGAP1 gene exhibited significantly lower levels of surface NR2A and impaired ERK activity compared to their wild-type littermates. Accordingly, primary hippocampal cultures of IQGAP1(-/-) neurons exhibited reduced surface expression of NR2A and disrupted ERK signaling in response to NR2A-dependent NMDAR stimulation. These molecular changes were accompanied by region-specific reductions of dendritic spine density in key brain areas involved in cognition, emotion, and motivation. IQGAP1 knock-outs exhibited marked long-term memory deficits accompanied by impaired hippocampal long-term potentiation (LTP) in a weak cellular learning model; in contrast, LTP was unaffected when induced with stronger stimulation paradigms. Anxiety- and depression-like behavior remained intact. On the basis of these findings, we propose that a dysfunctional IQGAP1 gene contributes to the cognitive deficits in brain disorders characterized by fewer dendritic spines.

A role for myosin Va in cerebellar plasticity and motor learning: a possible mechanism underlying neurological disorder in myosin Va disease

Mutations of the myosin Va gene cause the neurological diseases Griscelli syndrome type 1 and Elejalde syndrome in humans and dilute phenotypes in rodents. To understand the pathophysiological mechanisms underlying the neurological disorders in myosin Va diseases, we conducted an integrated analysis at the molecular, cellular, electrophysiological, and behavioral levels using the dilute-neurological (d-n) mouse mutant. These mice manifest an ataxic gait and clonic seizures during postnatal development, but the neurological disorders are ameliorated in adulthood. We found that smooth endoplasmic reticulum (SER) rarely extended into the dendritic spines of Purkinje cells (PCs) of young d-n mice, and there were few, if any, IP(3) receptors. Moreover, long-term depression (LTD) at parallel fiber-PC synapses was abolished, consistent with our previous observations in juvenile lethal dilute mutants. Young d-n mice exhibited severe impairment of cerebellum-dependent motor learning. In contrast, adult d-n mice showed restoration of motor learning and LTD, and these neurological changes were associated with accumulation of SER and IP(3) receptors in some PC spines and the expression of myosin Va proteins in the PCs. RNA interference-mediated repression of myosin Va caused a reduction in the number of IP(3) receptor-positive spines in cultured PCs. These findings indicate that myosin Va function is critical for subsequent processes in localization of SER and IP(3) receptors in PC spines, LTD, and motor learning. Interestingly, d-n mice had defects of motor coordination from young to adult ages, suggesting that the role of myosin Va in PC spines is not sufficient for motor coordination.

Selective Vulnerabilities of N-methyl-D-aspartate (NMDA) Receptors During Brain Aging

N-methyl-D-aspartate (NMDA) receptors are present in high density within the cerebral cortex and hippocampus and play an important role in learning and memory. NMDA receptors are negatively affected by aging, but these effects are not uniform in many different ways. This review discusses the selective age-related vulnerabilities of different binding sites of the NMDA receptor complex, different subunits that comprise the complex, and the expression and functions of the receptor within different brain regions. Spatial reference, passive avoidance, and working memory, as well as place field stability and expansion all involve NMDA receptors. Aged animals show deficiencies in these functions, as compared to young, and some studies have identified an association between age-associated changes in the expression of NMDA receptors and poor memory performance. A number of diet and drug interventions have shown potential for reversing or slowing the effects of aging on the NMDA receptor. On the other hand, there is mounting evidence that the NMDA receptors that remain within aged individuals are not always associated with good cognitive functioning. This may be due to a compensatory response of neurons to the decline in NMDA receptor expression or a change in the subunit composition of the remaining receptors. These studies suggest that developing treatments that are aimed at preventing or reversing the effects of aging on the NMDA receptor may aid in ameliorating the memory declines that are associated with aging. However, we need to be mindful of the possibility that there may also be negative consequences in aged individuals.

Unimpaired trace classical eyeblink conditioning in Purkinje cell degeneration (pcd) mutant mice

Young adult Purkinje cell degeneration (pcd) mutant mice, with complete loss of cerebellar cortical Purkinje cells, are impaired in delay eyeblink classical conditioning. In the delay paradigm, the conditioned stimulus (CS) overlaps and coterminates with the unconditioned stimulus (US), and the cerebellar cortex supports normal acquisition. The ability of pcd mutant mice to acquire trace eyeblink conditioning in which the CS and US do not overlap has not been explored. Recent evidence suggests that cerebellar cortex may not be necessary for trace eyeblink classical conditioning. Using a 500 ms trace paradigm for which forebrain structures are essential in mice, we assessed the performance of homozygous male pcd mutant mice and their littermates in acquisition and extinction. In contrast to results with delay conditioning, acquisition of trace conditioning was unimpaired in pcd mutant mice. Extinction to the CS alone did not differ between pcd and littermate control mice, and timing of the conditioned response was not altered by the absence of Purkinje cells during acquisition or extinction. The ability of pcd mutant mice to acquire and extinguish trace eyeblink conditioning at levels comparable to controls suggests that the cerebellar cortex is not a critical component of the neural circuitry underlying trace conditioning. Results indicate that the essential neural circuitry for trace eyeblink conditioning involves connectivity that bypasses cerebellar cortex.

Intrauterine growth restriction due to uteroplacental insufficiency decreased white matter and altered NMDAR subunit composition in juvenile rat hippocampi

Uteroplacental insufficiency (UPI), the major cause of intrauterine growth restriction (IUGR) in developed nations, predisposes to learning impairment. The underlying mechanism is unknown. Neuronal N-methyl-d-aspartate receptors (NMDARs) are critical for synaptogenesis and learning throughout life. We hypothesized that UPI-induced IUGR alters rat hippocampal NMDAR NR2A/NR2B subunit ratio and/or NR1 mRNA isoform expression and synaptic density at day 21 (P21). To test this hypothesis, IUGR was induced by bilateral uterine artery ligation of the late-gestation Sprague-Dawley dam. At P21, hippocampal NMDAR subunit mRNA and protein were measured, as were levels of synaptophysin. Neuronal, synaptic, and glial density in CA1, CA3, and dentate gyrus (DG) was assessed by immunofluorescence. IUGR increased NR1 mRNA isoform NR1-3a and 1-3b expression in both sexes. In P21 males, IUGR increased protein levels of NR1 C2′ and decreased NR1 C2, NR2A, and the NR2A-to-NR2B ratio, whereas in females, IUGR increased NR2B protein. In males, IUGR was associated with decreased myelin basic protein-to-neuronal nuclei ratio in CA1, CA3, and DG. We conclude that IUGR has sex-specific effects and that neither neuronal loss nor decreased synaptic density appears to account for the changes in NMDAR subunits. Rather, it is possible that synaptic NMDAR subunit composition is altered. Our results suggest that apparent recovery in the IUGR hippocampus may be associated with synaptic hyperexcitability. We speculate that the NMDAR plays an important role in IUGR-associated cognitive impairment.

Expression of NR2B in cerebellar granule cells specifically facilitates effect of motor training on motor learning

It is believed that gene/environment interaction (GEI) plays a pivotal role in the development of motor skills, which are acquired via practicing or motor training. However, the underlying molecular/neuronal mechanisms are still unclear. Here, we reported that the expression of NR2B, a subunit of NMDA receptors, in cerebellar granule cells specifically enhanced the effect of voluntary motor training on motor learning in the mouse. Moreover, this effect was characterized as motor learning-specific and developmental stage-dependent, because neither emotional/spatial memory was affected nor was the enhanced motor learning observed when the motor training was conducted starting at the age of 3 months old in these transgenic mice. These results indicate that changes in the expression of gene(s) that are involved in regulating synaptic plasticity in cerebellar granule cells may constitute a molecular basis for the cerebellum to be involved in the GEI by facilitating motor skill learning.

A spiking network model for passage-of-time representation in the cerebellum

In Pavlovian delay eyeblink conditioning, the cerebellum represents the passage-of-time (POT) between onsets of conditioned and unconditioned stimuli (CS and US, respectively). To study possible computational mechanisms of the POT representation we built a large-scale spiking network model of the cerebellum. Consistent with our previous rate-coding model, we found two conditions necessary for the present model to represent the POT with a dynamic population of active granule cells: (i) long temporal integration of input signals; and (ii) random recurrent connections between granule and Golgi cells. When these conditions were satisfied, a nonrecurrent sequence of active granule cell populations was generated in response to a CS and, conversely, the POT from the CS onset was able to be read out from the sequence. Specifically, simulated N-methyl-D-aspartate (NMDA) channels with a long decay time constant at granule and Golgi cells were responsible for the long temporal integration. Thus, blocking the NMDA channels or ablating Golgi cells impaired the POT representation. Simulated glomerulus structure made POT representation robust against noise in mossy fibre inputs. Long-term potentiation induced at mossy fibre synapses on granule cells also served to enhance the robustness. We reproduced some experimental results of Pavlovian delay eyeblink conditioning using the present model. These results suggest that the recurrent network in the granular layer and NMDA channels in granule and Golgi cells play an essential role in the timing mechanisms in the cerebellum, whereas the glomerulus serves to realize a robust representation of time.

Involvement of protein-tyrosine phosphatase PTPMEG in motor learning and cerebellar long-term depression

Although protein-tyrosine phosphorylation is important for hippocampus-dependent learning, its role in cerebellum-dependent learning remains unclear. We previously found that PTPMEG, a cytoplasmic protein-tyrosine phosphatase expressed in Purkinje cells (PCs), bound to the carboxyl-terminus of the glutamate receptor delta2 via the postsynaptic density-95/discs-large/ZO-1 domain of PTPMEG. In the present study, we generated PTPMEG-knockout (KO) mice, and addressed whether PTPMEG is involved in cerebellar plasticity and cerebellum-dependent learning. The structure of the cerebellum in PTPMEG-KO mice appeared grossly normal. However, we found that PTPMEG-KO mice showed severe impairment in the accelerated rotarod test. These mice also exhibited impairment in rapid acquisition of the cerebellum-dependent delay eyeblink conditioning, in which conditioned stimulus (450-ms tone) and unconditioned stimulus (100-ms periorbital electrical shock) were co-terminated. Moreover, long-term depression at parallel fiber-PC synapses was significantly attenuated in these mice. Developmental elimination of surplus climbing fibers and the physiological properties of excitatory synaptic inputs to PCs appeared normal in PTPMEG-KO mice. These results suggest that tyrosine dephosphorylation events regulated by PTPMEG are important for both motor learning and cerebellar synaptic plasticity.

Impaired discrimination learning in mice lacking the NMDA receptor NR2A subunit

N-Methyl-D-aspartate receptors (NMDARs) mediate certain forms of synaptic plasticity and learning. We used a touchscreen system to assess NR2A subunit knockout mice (KO) for (1) pairwise visual discrimination and reversal learning and (2) acquisition and extinction of an instrumental response requiring no pairwise discrimination. NR2A KO mice exhibited significantly retarded discrimination learning. Performance on reversal was impaired in NR2A KO mice during the learning phase of the task; with no evidence of heightened perseverative responses. Acquisition and extinction of an instrumental behavior requiring no pairwise discrimination was normal in NR2A KO mice. The present findings demonstrate a significant and selective deficit in discrimination learning following loss of NR2A.

Preclinical investigation of the functional effects of memantine and memantine combined with galantamine or donepezil

Combinations of drugs approved to treat Alzheimer's disease (AD) were tested in older rabbits with delay eyeblink classical conditioning, a form of associative learning severely impaired in AD. In Experiment 1 (n=49 rabbits), low doses (0.1, 0.5, 1.0, and 0.0 (vehicle) mg/kg) of memantine (Namenda) were tested. These three doses neither improved nor impaired acquisition at a statistically significant level. The 0.5 mg/kg dose had the greatest effect numerically and did not cause sensitization or habituation in explicitly unpaired controls. In Experiment 2 (n=56), doses of galantamine (Razadyne; 3.0 mg/kg) and donepezil (Aricept; 0.75 mg/kg) that had comparable magnitudes of cholinesterase inhibition were tested alone and in combination with 0.5 mg/kg memantine. Older rabbits treated with galantamine and with galantamine+memantine learned significantly better than vehicle-treated rabbits, but adding memantine did not improve learning over galantamine alone. Older rabbits treated with donepezil or a combination of memantine and donepezil did not learn significantly better than rabbits treated with vehicle. Galantamine has two mechanisms of action: mild cholinesterase inhibition and allosteric modulation of nicotinic acetylcholine receptors (nAChRs). When equated for cholinesterase inhibition, galantamine had significant efficacy in the eyeblink conditioning model system, but donepezil did not, indicating that modulation of nAChRs may be the mechanism that significantly ameliorates learning deficits in this model. In the absence of AD neuropathology in older rabbits, memantine had no efficacy alone or in combination with the other drugs.

Developmental regulation of cognitive abilities: modified composition of a molecular switch turns on associative learning

N-methyl-D-aspartate receptors (NMDARs) act as molecular coincidence detectors and allow for association or dissociation between pre- and postsynaptic neurons. NMDA receptors are central to remodeling of synaptic connections during postnatal development and associative learning abilities in adults. The ability to remodel neural networks is altered during postnatal development, possibly due to a change in the composition of NMDARs. That is, as forebrain systems (and cerebellum) develop, synaptic NR2B-containing NMDARs (NR2B-NMDARs) are replaced by NR2A-containing NMDARs (NR2A-NMDARs) and NR2B-NMDARs move to extrasynaptic sites. During the initial phase of the switch, synapses contain both NR2A- and NR2B-NMDARs and both long-term potentiation and long-term depression are enhanced. As NMDAR subunit expression decreases and NR2A-NMDARs come to predominate in the synapse, channel function and synaptic plasticity are reduced, and remodeling ability dissipates. The end result is a balance of plasticity and stability that is optimal for information processing and storage. Associative learning abilities involving different sensory modalities emerge sequentially, in accordance with synaptic maturation in related cortical and underlying brain structures. Thus, developmental alterations in NMDAR composition that occur at different ages in various brain structures may explain the protracted nature of the maturation of various associative learning abilities.

Modulation of NMDA receptors in the cerebellum. II. Signaling pathways and physiological modulators regulating NMDA receptor function

NMDA receptors in cerebellum have specific characteristics that make their function and modulation different from those of NMDA receptors in other brain areas. The properties of the NMDA receptor that modulate its function: Subunit composition, post-translational modifications and synaptic localization are summarized in an accompanying article. In this review we summarize how different signaling molecules modulate the function of NMDA receptors. The function of the receptors is modulated by the co-agonists glycine and serine and this modulation is different in cerebellum than in other areas. The NMDA receptor also has binding sites for polyamines that regulate its function. Other signaling molecules that modulate NMDA receptors function are: cAMP, neurotrophic factors such as BDNF, FGF-2 or neuregulins. These and other molecules allow an interplay between NMDA receptors and other receptors for neurotransmitters that may in this way modulate NMDA receptor function. This has been reported, for example, for metabotropic glutamate receptors. The expression and function of NMDA receptor is also modulated by synaptic activity, allowing an adaptation of the receptors function to the external inputs. NMDA receptors modulate important cerebral processes. NMDA receptors in different brain areas seem to modulate different processes. Cerebellar NMDA receptors play a special role in the modulation of motor learning and coordination. This is also briefly reviewed.

N-methyl-D-aspartate receptors play important roles in acquisition and expression of the eyeblink conditioned response in glutamate receptor subunit delta2 mutant mice

Classical eyeblink conditioning has been known to depend critically on the cerebellum. Apparently consistent with this, glutamate receptor subunit delta2 null mutant mice, which have serious morphological and functional deficiencies in the cerebellar cortex, are severely impaired in delay paradigm. However, these mutant mice successfully learn in trace paradigm, even in '0-trace paradigm,' in which the unconditioned stimulus starts just after the conditioned stimulus terminates. Our previous studies revealed that the hippocampus and the muscarinic acetylcholine receptors play crucial roles in 0-trace paradigm in glutamate receptor subunit delta2 null mutant mice unlike in wild-type mice, suggesting a large contribution of the forebrain to 0-trace conditioning in this type of mutant mice. In the present study, we investigated the role of N-methyl-D-aspartate receptors in 0-trace eyeblink conditioning in glutamate receptor subunit delta2 null mutant mice. Mice were injected intraperitoneally with the noncompetitive N-methyl-d-aspartate receptor antagonist (+)MK-801 (0.1mg/kg) or saline, and conditioned with 350-ms tone conditioned stimulus followed by 100-ms periorbital shock unconditioned stimulus. Glutamate receptor subunit delta2 null mutant mice that received (+)MK-801 injection exhibited a severe impairment in acquisition of the conditioned response, compared with the saline-injected glutamate receptor subunit delta2 null mutant mice. In contrast, wild-type mice were not impaired in acquisition of 0-trace conditioned response by (+)MK-801 injection. After the injection solution was changed from (+)MK-801 to saline, glutamate receptor subunit delta2 null mutant mice showed a rapid and partial recovery of performance of the conditioned response. On the other hand, when the injection solution was changed from saline to (+)MK-801, glutamate receptor subunit delta2 null mutant mice showed a marked impairment in expression of the pre-acquired conditioned response, whereas impairment of the expression was small in wild-type mice. Injection of (+)MK-801 had no significant effects on spontaneous eyeblink frequency or startle eyeblink frequency to the tone conditioned stimulus in either glutamate receptor subunit delta2 null mutant mice or wild-type mice. These results suggest that N-methyl-D-aspartate receptors play critical roles both in acquisition and expression of the conditioned response in 0-trace eyeblink conditioning in glutamate receptor subunit delta2 null mutant mice.

The N-methyl-D-aspartate (NMDA)-type glutamate receptor GluRepsilon2 is important for delay and trace eyeblink conditioning in mice

It has been proposed that the N-methyl-d-aspartate (NMDA)-type glutamate receptor (GluR) plays an important role in synaptic plasticity, learning, and memory. The four GluRepsilon (NR2) subunits, which constitute NMDA receptors with a GluRzeta (NR1) subunit, differ both in their expression patterns in the brain and in their functional properties. In order to specify the distinct participation of each of these subunits, we focused on the GluRepsilon2 subunits, which are expressed mainly in the forebrain. We investigated delay and trace eyeblink conditioning in GluRepsilon2 heterozygous mutant mice whose content of GluRepsilon2 protein was decreased to about half of that in wild-type mice. GluRepsilon2 mutant mice exhibited severe impairment of the attained level of conditioned response (CR) in the delay paradigm, for which the cerebellum is essential and modulation by the forebrain has been suggested. Moreover, GluRepsilon2 mutant mice showed no trend toward CR acquisition in the trace paradigm with a trace interval of 500 ms, in which the forebrain is critically involved in successful learning. On the other hand, the reduction of GluRepsilon2 proteins did not disturb any basic sensory and motor functions which might have explained the observed impairment. These results are different from those obtained with GluRepsilon1 null mutant mice, which attain a normal level of the CR but at a slower rate in the delay paradigm, and showed a severe impairment in the trace paradigm. Therefore, the NMDA receptor GluRepsilon2 plays a more critical role than the GluRepsilon1 subunit in classical eyeblink conditioning.

Trace eyeblink conditioning is hippocampally dependent in mice

The goal of this study was to determine whether trace eyeblink conditioning is a hippocampally dependent associative learning task in the mouse. First, we examined trace intervals of 0, 250, and 500 ms to determine a relatively long trace interval that would support eyeblink conditioning in young adult C57BL/6 mice. Mice rapidly acquired conditioned responses (CRs) with a 0-ms trace interval, acquired CRs with a 250-ms trace interval in approximately 2 days (2 sessions per day), and showed little acquisition with a 500-ms trace interval. Control mice were presented randomly unpaired stimuli and failed to show conditioning. We then determined the effect of lesioning dorsal hippocampal neurons on trace eyeblink conditioning. The hippocampus was injected bilaterally with vehicle (phosphate-buffered saline), 0.1% ibotenic acid, or 1% ibotenic acid. The vehicle group showed >60% CRs. The 0.1% group showed significantly fewer CRs (35-45%). The 1% group showed a level of CRs similar to that of the control mice. All the lesioned mice exhibited >60% CRs when subsequently trained with a 0-ms trace interval. A regression analysis indicated that the volume of area CA1 lesioned was more predictive of the behavioral impairment than the lesion volume of either CA3 or dentate gyrus, or even the total lesion volume. We conclude that dorsal hippocampal neurons play a critical role in eyeblink conditioning when a 250-ms trace interval is used with the C57BL/6 mouse, and that this paradigm will be useful for studying behavior and the in vivo and in vitro electrophysiology of hippocampal neurons in normal and transgenic or knockout mice.

In vitro classical conditioning of the turtle eyeblink reflex: approaching cellular mechanisms of acquisition

The classically conditioned eyeblink reflex is the best studied model for understanding the neural mechanisms that underlie learning and memory. Here, data from an in vitro model of the conditioned eyeblink reflex are summarized with the aim of shedding some light on potential cellular mechanisms that may underlie eyeblink classical conditioning. An isolated brainstem-cerebellum preparation from turtles was developed in which to study the synaptic circuitry of pathways involving the cerebellum, red nucleus and brainstem nuclei. A neural correlate of an eyeblink response recorded in the abducens nerve can be conditioned entirely in vitro by pairing trigeminal and auditory nerve stimulation. Conditioned abducens nerve responses (CRs) are not generated or sustained by unpaired stimuli and their long latencies, on the order of hundreds of milliseconds, support the interpretation that the CRs are not unconditioned responses. Ablation experiments show that CRs can be generated in brainstem preparations lacking a cerebellum or the medulla. However, the timing of the CRs are disrupted by removal of the cerebellar circuitry. Thus, a highly reduced in vitro brainstem preparation demonstrates acquisition of CRs but poor timing features. Recent experiments have focused on elucidating cellular mechanisms for CR acquisition in the brainstem blink circuitry. These studies show that NMDA-mediated synaptic mechanisms are required to generate CRs and that the level of conditioning is associated with the upregulation of GluR4-containing AMPA receptors in the abducens motor nuclei. Data from immunocytochemistry and physiological experiments using the calcium/calmodulin-dependent protein kinase II (CaMKII) inhibitor KN-93 suggest that CaMKII does not have a key role in mediating the induction or expression of abducens nerve CRs. It is hypothesized that GluR4-containing AMPA receptors in the abducens motor nuclei are targeted to auditory nerve synapses by an NMDA receptor-dependent process to strengthen the CS input during conditioning which results in the generation of CRs. Future studies will examine the synaptic localization of GluR4 and potential signal transduction pathways involved in in vitro conditioning. Moreover, the role feedback loops through the cerebellum and their role in CR timing will be a key issue to address using this preparation.

mGluR1 in cerebellar Purkinje cells is required for normal association of temporally contiguous stimuli in classical conditioning

In metabotropic glutamate receptor-subtype 1 (mGluR1)-null (mGluR1-/-) mice, cerebellar long-term depression (LTD) and several forms of memory are impaired. However, because mGluR1 is expressed in various brain regions in wild-type mice, it has been difficult to identify which type of memory depends on mGluR1 expressed in a given brain region. Furthermore, severe ataxia in mGluR1-/- mice complicated interpretation of the data from non-cerebellum-dependent tasks. We have generated mGluR1-rescue mice, which express mGluR1 only in Purkinje cells (PCs) of their cerebellum, by introducing the mGluR1alpha transgene into mGluR1-/- mice under the control of a PC-specific promoter. The mGluR1-rescue mouse has normal LTD and displays no apparent ataxia. Therefore, this mouse is the first animal model in which effects of mGluR1 deficiency outside PCs can be studied without cerebellar dysfunction. We used three eyeblink conditioning paradigms with different temporal specificities between conditioned stimulus (CS) and unconditioned stimulus (US). Delay conditioning, in which CS and US coterminate, was impaired in mGluR1-/- mice but normal in mGluR1-rescue mice. However, both strains of mice displayed severe impairment in trace conditionings, in which a stimulus-free interval of 250 or 500 ms intervened between CS and US. We also examined social transmission of food-preference and novel-object-recognition memory tests. In these tasks, mGluR1-rescue mice showed normal short-term but impaired long-term memory. We conclude that mGluR1 in PCs is indispensable for normal learning of association of temporally contiguous stimuli in associative conditioning. In contrast, mGluR1 in other cell types is required for associating discontiguous stimuli and long-term memory formation in nonspatial hippocampus-dependent learning.

Time-limited role of the hippocampus in the memory for trace eyeblink conditioning in mice

We examined the role of the hippocampus in memory retention after trace eyeblink conditioning in mice. After establishing the conditioned response (CR) in the trace paradigm, mice received a bilateral aspiration of the dorsal hippocampus and its overlying neocortex on the next day (1-day group) or after 4 weeks (4-week group). Control mice received a neocortical aspiration on the same schedule as the hippocampal-lesion group. After 2 weeks of recovery, these groups received additional conditioning for 3 days. Frequency of the CR of the 1-day group was as low as spontaneous values on the first day in the post-lesion session and never reached pre-surgical level during the post-lesion sessions, while that of the control group did reach pre-surgical level during the post-lesion sessions although there was a transient decline just after lesion. In contrast to the 1-day group, the 4-week-hippocampal lesion group retained the CR and showed a further increase, without significant difference from the control group. The temporal pattern of the CR also was unchanged by the hippocampal lesion 4 weeks after learning. These results suggest a time-limited role for the hippocampus in memory retention after trace conditioning in mice: the CR acquired recently requires an intact hippocampus for its retention, but the CR acquired remotely does not. This is similar to the result reported in rabbits. Therefore, the mechanism and time course of memory consolidation after trace eyeblink conditioning may be similar in mice and rabbits.

Mutation of NMDA receptor subunit epsilon 1: effects on audiogenic-like seizures induced by electrical stimulation of the inferior colliculus in mice

It has been shown that the N-methyl-D-asparate (NMDA) receptor in the inferior colliculus is involved in the induction of audiogenic seizures (AGS). In the present study we examined audiogenic-like seizure susceptibility in GluR epsilon 1 null KO adult mice (n=32) and wild-type adult mice (n=28) by electrically stimulating the inferior colliculus (IC). Threshold current intensities of the GluR epsilon 1 KO mice for wild running, clonic and tonic seizures were higher than those of wild-type mice. In addition, the incidence rates of each seizure syndrome in GluR epsilon 1 KO mice were lower than in wild-type mice at each current intensity. These results show that GluR epsilon 1 KO mice were more resistant to audiogenic-like seizures induced by stimulating the IC. Thus, our findings suggest that the GluR epsilon 1 subunit plays an important role in regulating AGS.

Age-dependent impairment of delay and trace eyeblink conditioning in mice

The age effect on classical eyeblink conditioning in unrestrained mice (C57BL/6J strain) was evaluated. Mice were trained at one of three age periods (8, 45-50 or 85-90 weeks). In the delay paradigm, significant learning deficits were evident in the 85-90 week-old group, but no deficits were observed in the behavior of the 45-50 week-old group. On the other hand, in the trace paradigm with a stimulus-free trace interval of 500 ms, significant deficits became apparent at the age of 45-50 weeks. These results indicate that trace eyeblink conditioning is more susceptible to age-related deterioration of memory in mice than delay eyeblink conditioning.

Effects of the noncompetitive NMDA receptor antagonist MK-801 on classical eyeblink conditioning in mice

N-methyl-D-aspartate (NMDA) receptors are involved in synaptic plasticity and play a critical role in learning and memory. We investigated the effects of the noncompetitive NMDA receptor antagonist (+)MK-801 on classical eyeblink conditioning of mice, using various interstimulus intervals between the conditioned stimulus (CS) and unconditioned stimulus (US). A tone was used for the CS and a periorbital shock was used for the US. In the delay paradigm, in which the US coterminated with the CS or started immediately after CS offset, the effect of (+)MK-801 (0.1mg/kg, i.p.) was a slight impairment in the acquisition of the conditioned response (CR). During subsequent CS-alone trials, the responses of (+)MK-801-injected mice were extinguished as easily as those of saline-injected mice. In the trace paradigm, (+)MK-801 impaired acquisition of the CR with a trace interval of 250 ms more than it did with a trace interval of 100 ms, and more than in the delay paradigm. (+)MK-801 injected after acquisition of 250-ms trace conditioning did not impair expression or extinction of the CR. These results suggest that NMDA receptors are involved in acquisition of the CR during longer trace interval conditioning more than during shorter trace interval conditioning or delay conditioning, and that their contribution to extinction is much smaller than their contribution to acquisition in mouse eyeblink conditioning.

In vitro eye-blink classical conditioning is NMDA receptor dependent and involves redistribution of AMPA receptor subunit GluR4

The classically conditioned vertebrate eye-blink response is a model in which to study neuronal mechanisms of learning and memory. A neural correlate of this response recorded in the abducens nerve can be conditioned entirely in vitro using an isolated brainstem-cerebellum preparation from the turtle by pairing trigeminal and auditory nerve stimulation. Here it is reported that conditioning requires that the paired stimuli occur within a narrow temporal window of <100 msec and that it is blocked by the NMDA receptor antagonist d,l-2-amino-5-phosphonovaleric acid. Moreover, there is a significant positive correlation between the levels of conditioning and greater immunoreactivity with the glutamate receptor 4 (GluR4) AMPA receptor subunit in the abducens motor nuclei, but not with NMDAR1 or GluR1. It is concluded that in vitro classical conditioning of an abducens nerve eye-blink response is generated by NMDA receptor-mediated mechanisms that may act to modify the AMPA receptor by increasing GluR4 subunits in auditory nerve synapses.

Long-trace interval eyeblink conditioning is impaired in mutant mice lacking the NMDA receptor subunit epsilon 1

To elucidate the role of the N-methyl-D-aspartate (NMDA) -type glutamate receptor subunit epsilon 1 (GluR epsilon 1) in classical eyeblink conditioning, delay and trace eyeblink conditioning were investigated in GluR epsilon 1-null mutant mice. In delay conditioning and short-trace interval conditioning with a trace interval of 250 ms, GluR epsilon 1 mutant mice attained a normal level of the conditioned response (CR), although acquisition was a little slower than in wild-type mice. In contrast, GluR epsilon 1 mutant mice exhibited severe impairment of the attained level of the CR and disturbed temporal pattern of CR expression in trace conditioning with a longer trace interval of 500 ms. These findings indicate that GluR epsilon 1 is essential for long-trace interval eyeblink conditioning. The impairments of the associative learning with a long temporal separation between the conditioned and unconditioned stimuli observed in the GluR epsilon 1 mutant mice could be attributed to an impairment of hippocampal long-term potentiation in this line of mutant mice.

Classical eyeblink conditioning in glutamate receptor subunit delta 2 mutant mice is impaired in the delay paradigm but not in the trace paradigm

In mice lacking glutamate receptor subunit delta 2 (GluR delta 2(-/-_ mice), cerebellar long-term depression (LTD) at the parallel fibre-Purkinje cell synapses is disrupted. Unlike the cerebellar LTD-deficient mice previously used for eyeblink conditioning, however, the abnormalities of the GluR delta 2(-/-) mice are restricted to the cerebellar cortex. In delay eyeblink conditionings (interstimulus interval of 252 and 852 ms), in which the conditioned stimulus (CS) overlaps temporally with a coterminating unconditioned stimulus (US), GluR delta 2(-/-) mice are severely impaired in learning, strongly supporting the hypothesis that cerebellar cortical LTD is essential for delay conditioning. In the trace paradigm, in which a stimulus-free trace interval of 500 ms intervened between the CS and US, GluR delta 2(-/-) mice learned as successfully as wild-type mice, indicating that cerebellar LTD is not necessary for trace conditioning. Thus, the present study has revealed a cerebellar LTD-independent learning in eyeblink conditioning.

Intra-cerebellar infusion of NMDA receptor antagonist AP5 disrupts classical eyeblink conditioning in rabbits

Rabbits were infused with AP5, an NMDA receptor antagonist, into the region of the cerebellar interpositus nucleus during classical eyeblink conditioning with a tone conditioned stimulus and an air puff unconditioned stimulus. Acquisition of the conditioned eyeblink response was delayed in rabbits infused with AP5 but the NMDA receptor antagonist had little effect on conditioned responses when these same rabbits were infused a second time after reaching asymptotic responding levels. Some rabbits that received AP5 infusions for the first time after the conditioned response was well learned showed temporary alterations in response timing. These data indicate that NMDA receptor activity is involved in the acquisition of classically conditioned eyeblink response and may also be involved in regulating cellular processes involved in response timing and other aspects of conditioned response execution.

Fetal alcohol exposure alters neurosteroid modulation of hippocampal N-methyl-D-aspartate receptors

The actions of ethanol on brain ligand-gated ion channels have important roles in the pathophysiology of alcohol-related neurodevelopmental disorders and fetal alcohol syndrome. Studies have shown that N-methyl-d-aspartate (NMDA) receptors are among the ligand-gated ion channels affected by prenatal ethanol exposure. We exposed pregnant dams to an ethanol-containing liquid diet that results in blood ethanol levels near the legal intoxication limit in most states (0.08%). Primary cultures of hippocampal neurons were prepared from the neonatal offspring of these dams, and NMDA receptor function was assessed by patch clamp electrophysiological techniques after 6-7 days in culture in ethanol-free media. Unexpectedly, we did not detect any changes in hippocampal NMDA receptor function at either the whole-cell or single-channel levels. However, we determined that fetal alcohol exposure alters the actions of the neurosteroids pregnenolone sulfate and pregnenolone hemisuccinate, which potentiate NMDA receptor function. Western immunoblot analyses demonstrated that this alteration is not due to a change in the expression levels of NMDA receptor subunits. Importantly, in utero ethanol exposure did not affect the actions of neurosteroids that inhibit NMDA receptor function. Moreover, the actions of pregnenolone sulfate on type A gamma-aminobutyric acid and non-NMDA receptor function were unaltered by ethanol exposure in utero, which suggests that the alteration is specific to NMDA receptors. These findings are significant because they provide, at least in part, a plausible mechanistic explanation for the alterations in the behavioral responses to neurosteroids found in neonatal rats prenatally exposed to ethanol and to other forms of maternal stress (Zimmerberg, B., and McDonald, B. C. (1996) Pharmacol. Biochem. Behav. 55, 541-547).

N-methyl-D-aspartate receptor subtype 2C is not involved in circadian oscillation or photoic entrainment of the biological clock in mice

Ishida et al. [1994: Neurosci Lett 166: 211-215] reported the circadian change of N-methyl-D-aspartate (NMDA) receptor subtype 2C mRNA and photic induction of this receptor's mRNA in the suprachiasmatic nucleus (SCN). Therefore, we investigated the role of NMDA receptor subtypes in the biological clock using NMDA receptor 2A (NR2A)- or 2C (NR2C)-deficient mice. However, NR2C-/- mice showed normal light-dark (LD)-entrained locomotor activity rhythms and free-running rhythms under constant darkness and also exhibited normal reentrainment to 6-hr LD shifts and phase delays with single light pulses. Thus, present results demonstrated no significant NR2C contribution to circadian oscillation and photic entrainment, even though expression of NR2C mRNA was highly observed in the SCN. On the other hand, the period of the free-running activity rhythm in NR2A-/- mice but not NR2C-/- mice was slightly longer than that in wild-type mice in spite of low expression of NR2A in the SCN. Furthermore, reentrainment to an LD advance in NR2A-/- mice was slower under low-intensity light conditions. Thus, we suggest that NR2A plays a role in determining the behavioral state that affects the circadian rhythm. In order to elucidate the role of NR2A and NR2C in the SCN, we examined NMDA-induced Ca(2+) elevations in the SCN of mutant mice using a Ca(2+) imaging method. A partial reduction in Ca(2+) elevation was observed in both NR2A-/- and NR2C-/- mice when high concentrations (100 or 300 microM) of NMDA were applied. The present results suggest that NR2A plays a weak role in oscillation or entrainment of the biological clock, and that NR2C does not participate in the functions of circadian oscillation and light entrainment.

Close linkage between calcium/calmodulin kinase II alpha/beta and NMDA-2A receptors in the lateral amygdala and significance for retrieval of auditory fear conditioning

The general mechanism underlying memory and learning is an area under intense investigation and debate, yet this mechanism still remains elusive. Auditory fear conditioning (when a tone is paired with a foot shock) is a simple associative form of learning for which many mechanistic details are known. Lesions of the lateral/basolateral nuclei of the amygdala result in the selective impairment of fear conditioning, indicating that this is a key region for this type of learning. Fear conditioning induces a lasting synaptic potentiation in the lateral nuclei of the amygdala. In addition, recent results from several laboratories suggest that N-methyl-D-aspartate (NMDA) receptor activation in the amygdala is required for the acquisition and expression of cue-conditioned fear responses using several kinds of antagonists. Little is known, however, about the signal transduction pathway and molecular substrate underlying fear conditioning. Here we use NMDA receptor-deficient mice to demonstrate that calmodulin-dependent kinase II, CaMKIIbeta, and CaMKIIalpha activation involves the NR2A subunit in the lateral/basolateral amygdala during memory retrieval following auditory fear conditioning. These results suggest that auditory fear conditioning involves a close linkage between NMDA2A receptors and the CaMKII cascade.

Dynamic characteristics and adaptability of reflex eye movements of Fyn-kinase-deficient mice

Fyn-kinase is expressed widely in the entire brain, including the cerebellum. Fyn-kinase-deficient mice are known to exhibit hypersensitivity to ethanol. To evaluate the cerebellar functions of Fyn-kinase, we examined the dynamic characteristics of the horizontal optokinetic response (HOKR) and vestibulo-ocular reflex (HVOR) and its adaptability in Fyn-kinase-deficient mice. The HOKR was induced by sinusoidal oscillation of a checkered screen and the HVOR was induced by sinusoidal oscillation of a turntable in darkness. The HOKR gains of mutant mice were higher than those of the wild-type mice, and the HVOR phases of mutant mice were less advanced than those of the wild-type mice. However, no difference was noted in the adaptability of the HOKR induced by 1 h of sustained screen oscillation between the mutant and wild-type mice. The cerebellar functions appear to be unaffected by Fyn-kinase knockout.

Genetic regulation of glutamate receptor ion channels

Transcriptional and translational regulation of glutamate receptor expression determines one of the key phenotypic features of neurons in the brain--the properties of their excitatory synaptic receptors. Up- and down-regulation of various glutamate receptor subunits occur throughout development, following ischemia, seizures, repetitive activation of afferents, or chronic administration of a variety of drugs. The promoters of the genes that encode the NR1, NR2B, NR2C, GluR1, GluR2, and KA2 subunits share several characteristics that include multiple transcriptional start sites within a CpG island, lack of TATA and CAAT boxes, and neuronal-selective expression. In most cases, the promoter regions include overlapping Sp1 and GSG motifs near the major initiation sites, and a silencer element, to guide expression in neurons. Manipulating the levels of glutamate receptors in vivo by generating transgenic and knockout mice has enhanced understanding of the role of specific glutamate receptor subunits in long-term potentiation and depression, learning, seizures, neural pattern formation, and survival. Neuron-specific glutamate receptor promoter fragments may be employed in the design of novel gene-targeting constructs to deliver future experimental transgene and therapeutic agents to selected neurons in the brain.