Genetic enhancement of learning and memory in mice

PDZ domain suppression of an ER retention signal in NMDA receptor NR1 splice variants

The NMDA receptor NR1 subunit has four splice variants that differ in their C-terminal, cytoplasmic domain. We investigated the contribution of the C-terminal cassettes, C0, C1, C2, and C2', to trafficking of NR1 in heterologous cells and neurons. We identified an ER retention signal (RRR) in the C1 cassette of NR1, which is similar to the RXR motif in ATP-sensitive K(+) channels (Zerangue et al., 1999). We found that surface expression of NR1-3, which contains C1, is due to a site on the C2' cassette, which includes the terminal 4 amino acid PDZ-interacting domain. This site suppresses ER retention of the C1 cassette and leads to surface expression. These findings suggest a role for PDZ proteins in facilitating the transition of receptors from an intracellular pool to the surface of the neuron.

Neuroendocrine aging in the female rat: the changing relationship of hypothalamic gonadotropin-releasing hormone neurons and N-methyl-D-aspartate receptors

The reproductive axis undergoes alterations during aging, resulting in acyclicity and the loss of reproductive function. In the hypothalamus, changes intrinsic to GnRH neurons may play a critical role in this process, as may changes in inputs to GnRH neurons from neurotransmitters such as glutamate. We investigated the effects of age and reproductive status on neuroendocrine glutamatergic NMDA receptors (NRs), their regulation of GnRH neurons, and their expression on GnRH neurons, in female rats. First, we quantified NR subunit messenger RNAs (mRNAs) in preoptic area-anterior hypothalamus (POA-AH) and medial basal hypothalamus (MBH), the sites of GnRH perikarya and neuroterminals, respectively. In POA-AH, NR1 mRNA levels varied little with age or reproductive status. NR2a and NR2b mRNA levels decreased significantly between cycling and acyclic rats. In MBH, NR mRNAs all increased with aging, particularly in acyclic animals. Second, we tested the effects of N-methyl-D,L-aspartate (NMA) on GnRH mRNA levels in POA-AH of aging rats. NMA elevated GnRH mRNA levels in young rats, but decreased them in middle-aged rats. Third, we quantified expression of the NR1 subunit on GnRH perikarya in aging rats using double label immunocytochemistry. NR1 expression on GnRH cell bodies varied with age and reproductive status, with 30%, 19%, and 46% of GnRH somata double labeled with NR1 in young proestrous, middle-aged proestrous, and middle-aged persistent estrous rats, respectively. Thus, 1) the expression of hypothalamic NR subunit mRNAs correlates with reproductive status; 2) changes in NR subunit mRNA levels, if reflected by changes in protein levels, may result in alterations in the stoichiometry of the NR during aging, with possible physiological consequences; 3) the effects of NR activation on GnRH mRNA switches from stimulatory to inhibitory during reproductive aging; and 4) expression of the NR1 subunit on GnRH perikarya changes with reproductive status. These molecular, physiological, and cellular neuroendocrine changes are proposed to be involved in the transition to acyclicity in aging female rats.

Differential participation of the NBM in the acquisition and retrieval of conditioned taste aversion and Morris water maze

Deficits in both learning and memory after lesions of the cholinergic basal forebrain, in particular the nucleus basalis magnocellularis (NBM), have been widely reported. However, the participation of the cholinergic system in either acquisition or retrieval of memory process is still unclear. In this study, we tested the possibility that excitotoxic lesions of the NBM affect either acquisition or retrieval of two tasks. In the first experiment, animals were trained for two conditioned taste aversion tasks using different flavors, saccharine and saline. The acquisition of the first task was before NBM lesions (to test retrieval) and the acquisition of the second task was after the lesions (to test acquisition). Accordingly, in the first part of the second experiment, animals were trained in the Morris water maze (MWM), lesioned and finally tested. In the final part of this experiment, another set of animals was lesioned, then trained in the MWM and finally tested. All animals were able to retrieve conditioned taste aversion (CTA) and MWM when learned before NBM lesions; however, lesions disrupted the acquisition of CTA and MWM. The results suggest that the NBM and cholinergic system may play an important role in acquisition but not during retrieval of aversive memories.

Enhancement of memory-related long-term facilitation by ApAF, a novel transcription factor that acts downstream from both CREB1 and CREB2

The memory for sensitization of the gill withdrawal reflex in Aplysia is reflected in facilitation of the monosynaptic connection between the sensory and motor neurons of the reflex. The switch from short- to long-term facilitation requires activation of CREB1, derepression of ApCREB2, and induction of ApC/EBP. In search for genes that act downstream from CREB1, we have identified a transcription activator, ApAF, which is stimulated by protein kinase A and can dimerize with both ApC/EBP and ApCREB2. ApAF is necessary for long-term facilitation induced by five pulses of serotonin, by activation of CREB1, or by derepression of ApCREB2. Overexpression of ApAF enhances the long-term facilitation further. Thus, ApAF is a candidate memory enhancer gene downstream from both CREB1 and ApCREB2.

NMDA receptor-dependent synaptic reinforcement as a crucial process for memory consolidation

The hippocampal CA1 region is crucial for converting new memories into long-term memories, a process believed to continue for week(s) after initial learning. By developing an inducible, reversible, and CA1-specific knockout technique, we could switch N-methyl-D-aspartate (NMDA) receptor function off or on in CA1 during the consolidation period. Our data indicate that memory consolidation depends on the reactivation of the NMDA receptor, possibly to reinforce site-specific synaptic modifications to consolidate memory traces. Such a synaptic reinforcement process may also serve as a cellular means by which the new memory is transferred from the hippocampus to the cortex for permanent storage.

Cognition and synaptic plasticity in diabetes mellitus

Diabetes mellitus is associated with cognitive deficits and an increased risk of dementia, particularly in the elderly. These deficits are paralleled by neurophysiological and structural changes in the brain. In animal models of diabetes, impairments of spatial learning occur in association with distinct changes in hippocampal synaptic plasticity. At the molecular level these impairments might involve changes in glutamate-receptor subtypes, in second-messenger systems and in protein kinases. The multifactorial pathogenesis of diabetic encephalopathy is not yet completely understood, but clearly shares features with brain ageing and the pathogenesis of diabetic neuropathy. It involves both metabolic and vascular changes, related to chronic hyperglycaemia, but probably also defects in insulin action in the brain. Treatment with insulin might therefore not only correct hyperglycaemia, but could also directly affect the brain.

Glucocorticoids and the ageing hippocampus

Approximately 30 % of human and mammalian populations develop cognitive impairments with ageing. Many of these impairments have been linked to dysfunction of the hippocampus, a well studied area of the medial-temporal lobe, which is involved in episodic memory and control of the hypothalamo-pituitary-adrenal stress axis and, thus, of glucocorticoid secretion. This paper reviews the growing body of studies which explore a possible relationship between lifetime exposure to glucocorticoids and hippocampal impairment. There is now strong evidence which associates hypercortisolemia in aged men with later cognitive dysfunction and this complements a wealth of rodent and other human data. We conclude with a discussion of possible pharmacological and behavioural interventions.

Behavioural pharmacology and its contribution to the molecular basis of memory consolidation

Recent findings have significantly advanced our understanding the mechanisms of memory formation. Most of these advances stemmed from behavioural pharmacology research involving, particularly, the localized infusion of drugs with specific molecular actions into specific brain regions. This approach has revealed brain structures involved in different memory types and the main neurotransmitter systems and sequence of metabolic cascades that participate in memory consolidation. Biochemical studies and, in several cases, studies of genetically manipulated animals, in which receptors or enzymes affected by the various drugs were absent or overexpressed, have complemented the pharmacological research. Although most studies have concentrated on the involvement of the hippocampus, many have also investigated the entorhinal cortex, other regions of the cortex, and the amygdala. Behavioural pharmacology has been of crucial importance in establishing the major neurohumoral and hormonal systems involved in the modulation of memory formation. These systems act on specific steps of memory formation in the hippocampus and in the entorhinal, parietal, and cingulate cortex. A specialized system mediated by the basolateral amygdaloid nucleus, and involving several neuromodulatory systems, is activated by emotional arousal and serves to regulate memory formation in other brain regions. The core mechanisms involved in the formation of explicit (declarative) memory are in many respects similar to those of long-term potentiation (LTP), particularly in the hippocampus. However, there are also important differences between memory formation and LTP. Memory formation involves numerous modulatory influences, the co-participation of various brain regions other than the hippocampus, and some properties that are specific to memory and absent in LTP (i.e. flexibility of response). We discuss the implications of these similarities and differences for understanding the neural bases of memory.

DNA

The authors predict that in a few years, many areas of psychology will be awash in specific genes responsible for the widespread influence of genetics on behavior. As the focus shifts from finding genes (genomics) to understanding how genes affect behavior (behavioral genomics), it is important for the future of psychology as a science that pathways between genes and behavior be examined not only at the molecular biological level of cells or the neuroscience level of the brain but also at the psychological level of analysis. After a brief overview of quantitative genetic research, the authors describe how genes that influence complex traits like behavioral dimensions and disorders in human and nonhuman animals are being found. Finally, the authors discuss behavioral genomics and predict that DNA will revolutionize psychological research and treatment early in the 21st century.

Regulation of NMDA receptor subunits and nitric oxide synthase expression during cocaine withdrawal

The present study characterized the effects of withdrawal from cocaine on the expression of NMDA receptor subunits (NR1, NR2B) and neuronal nitric oxide synthase. FosB induction was measured to confirm that repeated cocaine exposure influenced protein expression, as previously reported. Administration of cocaine followed by 24 h, 72 h, or 14 days of withdrawal resulted in alterations of NR1 and NR2B subunits and neuronal nitric oxide synthase expression as measured by immunohistochemical labeling of rat brain sections. Optical density analyses revealed significant up-regulation of NR1 in the ventral tegmental area at 72 h and 14 days of withdrawal. Structure-specific and withdrawal time-dependent alterations in NR2B expression were also found. After 24 h of withdrawal, cocaine-induced decreases in NR2B expression were observed in the nucleus accumbens shell, whereas increases in NR2B expression were found in medial cortical areas. Two weeks of withdrawal from cocaine caused an approximately 50% increase in NR2B subunit expression in regions of the cortex, neostriatum, and nucleus accumbens. In contrast, cocaine-induced up-regulation of neuronal nitric oxide synthase was transient and evident in cortical areas only at 24 h after the last drug injection. The results suggest that region-specific changes in interactions among proteins associated with the NMDA receptor complex may underlie neuronal adaptations following repeated cocaine administration.

Synaptic activity-dependent developmental regulation of NMDA receptor subunit expression in cultured neocortical neurons

The biophysical properties of NMDA receptors are thought to be critical determinants involved in the regulation of long-term synaptic plasticity during neocortical development. NMDA receptor channel properties are strongly dependent on the subunit composition of heteromeric NMDA receptors. During neocortical development in vivo, the expression of the NMDA receptor 2A (NR2A) subunit is up-regulated at the mRNA and protein level correlating with changes in the kinetic and pharmacological properties of functional NMDA receptors. To investigate the developmental regulation of NMDA receptor subunit expression, we studied NR2 mRNA expression in cultured neocortical neurons. With increasing time in culture, they showed a similar up-regulation of NR2A mRNA expression as described in vivo. As demonstrated by chronic blockade of postsynaptic glutamate receptors in vitro, the regulation of NR2A mRNA was strongly dependent on synaptic activity. In contrast, NR2B mRNA expression was not influenced by activity blockade. Moreover, as shown pharmacologically, the regulation of NR2A mRNA expression was mediated by postsynaptic Ca(2+) influx through both NMDA receptors and L-type Ca(2+) channels. It is interesting that even relatively weak expression of NR2A mRNA was correlated with clearly reduced sensitivity of NMDA receptor-mediated whole-cell currents against the NR2B subunit-specific antagonist ifenprodil. Developmental changes in the expression of NR1 mRNA splice variants were also strongly dependent on synaptic activity and thus might, in addition to regulation of NR2 subunit expression, contribute to developmental changes in the properties of functional NMDA receptors. In summary, our results demonstrate that synaptic activity is a key factor in the regulation of NMDA receptor subunit expression during neocortical development.

Behavioral neuroscience: challenges for the era of molecular biology

This is a great age to participate in biological inquiry. Behavioral neuroscience offers a rich perspective for molecular biologists. However, behavioral analysis is not simply an assay. Whereas molecular biology has become a unique tool in the armamentarium of behavioral neuroscience, the powerful methodology of molecular biology is no substitute for careful behavioral exploration.

Activity-dependent induction of tonic calcineurin activity mediates a rapid developmental downregulation of NMDA receptor currents

Whole-cell recording in the superficial layers of the developing superior colliculus (sSC) reveals a large drop in NMDA receptor (NMDAR) current decay time synchronized across all neurons and occurring consistently between P10 and P11. We show that blocking the Ca2+/calmodulin-dependent phosphatase calcineurin (CaN) in the postsynaptic neuron can abolish this drop. The regulation is induced prematurely by 1-2 hr of electrical stimulation in P10 collicular slices only if CaN and NMDAR currents can be activated in the neuron. These data suggest that a long-lasting, CaN-mediated control of NMDAR kinetics is rapidly initiated by heightened activity of the NMDAR itself and demonstrate a novel developmental and tonic function of CaN that can play an important role in modulating the plasticity of the developing CNS.

Cognitive neuroscience

The last decade of the 20th century has seen the development of cognitive neuroscience as an effort to understand how the brain represents mental events. We review the areas of emotional and motor memory, vision, and higher mental processes as examples of this new understanding. Progress in all of these areas has been swift and impressive, but much needs to be done to reveal the mechanisms of cognition at the local circuit and molecular levels. This work will require new methods for controlling gene expression in higher animals and in studying the interactions between neurons at multiple levels.

Antisense DNA against calcineurin facilitates memory in contextual fear conditioning by lowering the threshold for hippocampal long-term potentiation induction

In the previous study, we demonstrated that the antisense oligodeoxynucleotides against calcineurin Aalpha and Abeta, catalytic subunits of Ca(2+)/calmodulin-dependent protein phosphatase, produce a facilitatory effect on long-term potentiation induction in the hippocampal CA1 region in rats anesthetized with urethane. Here, we have studied how animals, in which the hippocampal long-term potentiation induction is enhanced by antisense oligodeoxynucleotides against calcineurin, perform in learning tasks that depend on hippocampal function. The rats received antisense oligodeoxynucleotides by bilateral ventricular administration via miniosmotic pumps. We tested four groups of rats, three infused with either antisense oligodeoxynucleotides, scramble oligodeoxynucleotides, or saline, and untreated rats, for two types of hippocampus-dependent learning, water maze and contextual fear conditioning. After the behavioral tests, we conducted a long-term potentiation induction test to determine whether long-term potentiation induction was enhanced. In contextual fear conditioning, rats in which long-term potentiation induction was enhanced by antisense oligodeoxynucleotides displayed significantly more conditioned freezing response than control rats. Rats with enhanced long-term potentiation induction showed no differences in shock sensitivity, general activity, or light-dark choice from control rats. In contrast with contextual fear conditioning, rats with enhanced long-term potentiation induction showed no difference in spatial learning performance on the water maze compared with control rats. These results demonstrate that an enhancement in long-term potentiation induction produced by the inhibition of calcineurin leads to an increase in memory strength in specific forms of hippocampus-dependent learning.

Synaptic plasticity and memory: an evaluation of the hypothesis

Changing the strength of connections between neurons is widely assumed to be the mechanism by which memory traces are encoded and stored in the central nervous system. In its most general form, the synaptic plasticity and memory hypothesis states that "activity-dependent synaptic plasticity is induced at appropriate synapses during memory formation and is both necessary and sufficient for the information storage underlying the type of memory mediated by the brain area in which that plasticity is observed." We outline a set of criteria by which this hypothesis can be judged and describe a range of experimental strategies used to investigate it. We review both classical and newly discovered properties of synaptic plasticity and stress the importance of the neural architecture and synaptic learning rules of the network in which it is embedded. The greater part of the article focuses on types of memory mediated by the hippocampus, amygdala, and cortex. We conclude that a wealth of data supports the notion that synaptic plasticity is necessary for learning and memory, but that little data currently supports the notion of sufficiency.

Intracerebroventricular injection of the N-terminal substance P fragment SP(1-7) regulates the expression of the N-methyl-D-aspartate receptor NR1, NR2A and NR2B subunit mRNAs in the rat brain

Studies have implicated substance P (SP) in the regulation of affective behaviour, memory function, pain influx, stress and opioid reward. All these dimensions are known to involve glutamate transmission mediated through the N-methyl-D-aspartate (NMDA) receptor. The SP N-terminal fragment SP(1-7) is shown to share some but oppose other effects of the parent compound. We have examined the effect of intracerebroventricular injections of SP(1-7) on the expression of the NMDA receptor subunits NR1, NR2A and NR2B mRNAs in the spinal cord and in discrete areas of the male rat brain. The results indicated that the heptapeptide induced a dose-dependent upregulation of the NR2A transcript in hippocampus, periaqueductal grey and ventral tegmental area, already within a few hours. The level of the NR2B mRNA was increased in hippocampus and nucleus accumbens. The expression of the transcript of the NR1 was enhanced in hippocampus and nucleus accumbens but attenuated in spinal cord. The observed effects of the SP(1-7) fragment are in agreement with what could be expected from the known effects of the heptapeptide on various behaviours involving glutamate transmission.

Competitive Hebbian learning through spike-timing-dependent synaptic plasticity

Hebbian models of development and learning require both activity-dependent synaptic plasticity and a mechanism that induces competition between different synapses. One form of experimentally observed long-term synaptic plasticity, which we call spike-timing-dependent plasticity (STDP), depends on the relative timing of pre- and postsynaptic action potentials. In modeling studies, we find that this form of synaptic modification can automatically balance synaptic strengths to make postsynaptic firing irregular but more sensitive to presynaptic spike timing. It has been argued that neurons in vivo operate in such a balanced regime. Synapses modifiable by STDP compete for control of the timing of postsynaptic action potentials. Inputs that fire the postsynaptic neuron with short latency or that act in correlated groups are able to compete most successfully and develop strong synapses, while synapses of longer-latency or less-effective inputs are weakened.

Generation of mouse mutants as a tool for functional genomics

As sequence information becomes available from the Human Genome Project, key developments include systematic methods for assigning function to each of the 100,000 or so genes. Strategies for coping with this sequence information, including microarray analysis and proteomics, will further our understanding of how genes function and interact. Ultimately, however, the simplest way to understand how a gene works is to examine the consequences of interference with its function: mutational analysis. The mouse represents the model organism of choice in the analysis of gene function; close enough to human to represent a satisfactory model organism, yet relatively easy to manipulate at a genetic level. Two complementary approaches, genotype- and phenotype-based, have been established in the mouse genetics and genomics communities to systematically generate new mouse mutations. Genotype-based approaches are advantageous in that molecular analysis of mutations is facilitated. Phenotypic analysis, however, is often assumed based on gene expression patterns, often leading to unexpected results. Phenotype-based approaches do not make prior assumptions about gene function. Often, however, it may be difficult to define the underlying genetic lesion. Progress in each of these approaches will be considered and situations in which they might be mutually beneficial will be investigated.

Maternal care, hippocampal synaptogenesis and cognitive development in rats

We report that variations in maternal care in the rat promote hippocampal synaptogenesis and spatial learning and memory through systems known to mediate experience-dependent neural development. Thus, the offspring of mothers that show high levels of pup licking and grooming and arched-back nursing showed increased expression of NMDA receptor subunit and brain-derived neurotrophic factor (BDNF) mRNA, increased cholinergic innervation of the hippocampus and enhanced spatial learning and memory. A cross-fostering study provided evidence for a direct relationship between maternal behavior and hippocampal development, although not all neonates were equally sensitive to variations in maternal care.

Multivariate analysis of associations of 42 genes in ADHD, ODD and conduct disorder

In a previous study (Comings DE et al. Comparison of the role of dopamine, serotonin, and noradrenergic genes in ADHD, ODD and conduct disorder. Multivariate regression analysis of 20 genes. Clin Genet 2000: 57: 178-196) we examined the role of 20 dopamine, serotonin and norepinephrine genes in attention deficit hyperactivity disorder (ADHD), oppositional defiant disorder (ODD), and conduct disorder (CD), using a multivariate analysis of associations (MAA) technique. We have now brought the total number of genes examined to 42 by adding an additional 22 candidate genes. These results indicate that even with the inclusion of these additional genes the noradrenergic genes still played a greater role in ADHD than any other group. Six other neurotransmitter genes were included in the regression equation - cholinergic, nicotinic, alpha 4 receptor (CHNRA4), adenosine A2A receptor (ADOA2A), nitric oxide synthase (NOS3), NMDAR1, GRIN2B, and GABRB3. In contrast to ADHD and ODD, CD preferentially utilized hormone and neuropeptide genes These included CCK, CYP19 (aromatase cytochrome P-450), ESR1, and INS (p = 0.005). This is consistent with our prior studies indicating a role of the androgen receptor (AR) gene in a range of externalizing behavors. We propose that the MAA technique, by focusing on the additive effect of multiple genes and on the cummulative effect of functionally related groups of genes, provides a powerful approach to the dissection of the genetic basis of polygenic disorders.

Early sensory and hormonal experience modulate age-related changes in NR2B mRNA within a forebrain region controlling avian vocal learning

Male zebra finches are most apt to mimic songs heard between posthatch days (PHD) 35 and 65, and this vocal learning depends, in part, on the activation of N-methyl-D-aspartate receptors (NMDAR) within a discrete forebrain circuit that includes the lateral magnocellular nucleus of the anterior neostriatum (lMAN) and area X. Using in situ hybridization, we show that transcripts for both the constitutive NMDAR subunit NR1 and the modulatory subunit NR2B decrease abruptly in the lMAN between PHD20 and 40. This downregulation corresponds to the onset of song learning and a transition from slow to faster NMDAR currents in lMAN neurons. In area X, NR1 mRNA increases as NR2B mRNA decreases during song development. To understand how these changes in NMDAR mRNA might regulate song learning, we next investigated how manipulations that influence song development affect NMDAR mRNA expression. Early isolation from conspecific song (which delays closure of the sensitive period for song learning) selectively increases NR2B, but not NR1 mRNA, within lMAN at PHD60. In contrast, exposure to testosterone beginning at PHD20 (which impairs song development and hastens the developmental transition to faster NMDAR current kinetics within lMAN) accelerates the decline in NR2B mRNA in lMAN, again without affecting NR1 transcript levels. Neither manipulation significantly effects NR1 or NR2B mRNA levels in area X. Our data suggest that developmental changes in the expression of specific NMDAR subunits may regulate periods of neural and behavioral plasticity and that flexibility in the timing of these sensitive periods may be achieved through experience and/or hormone-dependent modulation of NMDAR gene expression.

Development of NR1, NR2A and NR2B mRNA in NR1 immunoreactive cells of rat visual cortex

In cells marked for N-methyl-D-aspartate receptors (NMDARs), we studied the relationship between the sensitive period for monocular deprivation and the expression of rat NMDAR subunits, NR2A and NR2B. In the rat the sensitive period ends sometime after postnatal day 50 (P50). Previous studies of the development of these subunit mRNAs focused on animals prior to the end of the sensitive period and did not examine the visual cortex specifically. We used a monoclonal antibody to the NR1 subunit of the receptor to identify cells containing NMDARs. We then used in situ hybridization to label the same sections for NR2A or NR2B mRNA. In an additional experiment we labeled sections for NR1 mRNA to see if the developmental profile was similar at both the mRNA and protein level. We used five animals at each of four ages: P22, P30, P45 and P90. Staining for NR2B mRNA, but not for NR2A mRNA, decreased dramatically from P22 to P45. Staining for NR1 mRNA declined dramatically between P22 and P45 even though most cells remained strongly immunopositive for the NR1 protein during this time. This discrepancy suggests that significant NR1 regulation occurs after gene transcription. Because most of the decrease in NR1 mRNA and NR2B mRNA occurs by P30, transcriptional regulation of these subunits does not easily explain the loss of sensitivity to monocular deprivation, which occurs around P50. The changes are, in fact, more closely synchronized with the beginning of experience-dependent plasticity than with its end.

Enhanced learning after genetic overexpression of a brain growth protein

Ramón y Cajal proposed 100 years ago that memory formation requires the growth of nerve cell processes. One-half century later, Hebb suggested that growth of presynaptic axons and postsynaptic dendrites consequent to coactivity in these synaptic elements was essential for such information storage. In the past 25 years, candidate growth genes have been implicated in learning processes, but it has not been demonstrated that they in fact enhance them. Here, we show that genetic overexpression of the growth-associated protein GAP-43, the axonal protein kinase C substrate, dramatically enhanced learning and long-term potentiation in transgenic mice. If the overexpressed GAP-43 was mutated by a Ser --> Ala substitution to preclude its phosphorylation by protein kinase C, then no learning enhancement was found. These findings provide evidence that a growth-related gene regulates learning and memory and suggest an unheralded target, the GAP-43 phosphorylation site, for enhancing cognitive ability.

Evolution proposes and ontogeny disposes

Genes, the basic building blocks of evolution, are highly conserved. For example, the mouse and human have approximately the same number of genes, and around 94% are identical in the two species. Since species differ on multiple dimensions (e.g., anatomy, physiology, and behavior), it follows that identical genes may subserve different functions in different species. Two reasons for this are gene-gene interaction and gene-environment interaction (and it is the presence of these interactions which prevents one from making deterministic statements about genetics, thus rendering obsolete the nature-nurture controversy). Behavioral examples of both types of interactions are presented, including studies showing that (1) the uterine environment enhances later cognitive competence, (2) early postnatal experiences affect learning and emotionality and can extend into future generations, (3) maternal behavior changes the offspring's later behavior and physiology, and (4) knocking out one gene results in an animal less competent in one learning process but more competent in a complementary learning process.

DNA fragmentation factor 45 deficient mice exhibit enhanced spatial learning and memory compared to wild-type control mice

Programmed cell death or apoptosis is a highly regulated physiological process that is critical in development, particularly in the central nervous system. The DNA fragmentation factor 45 (DFF45 or ICAD) is a subunit of a heterodimeric DNase complex that is crucial for DNA fragmentation and normal apoptosis. To examine the neurobiological consequences of lacking DNA fragmentation and timely apoptosis during mouse development in vivo, we compared spatial learning behaviors in DFF45 mutant and wild-type control mice. We found that DFF45 mutant mice exhibit enhanced spatial learning and memory compared to wild-type mice. Moreover, both the granule cell density and total granule cell number in the hippocampal dentate gyrus region are higher in the DFF45 mutant brains than in the wild-type brains. We propose that the increase in granule cell number in the dentate region due to the DFF45 mutation changes the neuronal network underlying spatial learning and memory in DFF45 mutant mice.

Some new directions for research on the biology of aging

A highly selective, eclectic, and personal view of new directions and new opportunities for research on the biology of aging is briefly outlined. Some concern is raised regarding the present emphasis on the use of centenarians for the definition of genetic loci responsible for unusually robust retention of structure and function. More progress is likely to be made were we to focus on the genetic basis for "elite" aging in middle-aged subjects examined for very specific phenotypes, as these are likely to be far less polygenic. Descriptive gerontology is entering a renaissance, given such new clinical tools as functional MRI and basic science tools such as functional genomics and proteomics. Advances in genomics should expedite answers to such questions as why some avian species have exceptionally long lifespans despite unusual loads of oxidative stress. One hopes to see renewed mechanistic studies, using such tools, at the systems levels. New methodologies are permitting the evaluation of stochastic alterations in gene structure and function in postreplicative cells. The exciting work on molecular misreading should prompt us to reexplore the Orgel hypothesis as it applies to such cell types. Epigenetic shifts in gene expression that occur in association with sexual maturation and the cessation of growth may have deleterious consequences late in the life course. It will therefore be important for gerontologists to investigate the molecular biology of pubescence. Finally, our community should investigate the impact of environmental "gerontogens," agents that accelerate specific processes of aging and specific senescent phenotypes.

Electrophysiological analysis of NMDA receptor subunit changes in the aging mouse cortex

NMDA receptors play an important role in memory processes and plasticity in the brain. We have previously demonstrated a significant decrease in NMDARepsilon2 subunit mRNA and protein with increasing age in the C57Bl/6 mouse frontal cortex. In the present study, two-electrode voltage clamp electrophysiology on Xenopus oocytes injected with total RNA harvested from the frontal cortex of young and old C57Bl mice was used to detect changes in receptor composition during aging. Ifenprodil concentration-response curves, magnesium current-voltage curves, and single channel conductances were determined for native receptors. In addition, ifenprodil and magnesium curves were generated for recombinant NMDA receptors of varying subunit ratios. Ifenprodil dose-response curves for all receptors were biphasic. The low affinity component of the curve increased slightly with age, while the high affinity population decreased, mimicking recombinant receptors with decreasing levels of epsilon2. A decrease in maximal current was also observed in aged animals with decreased levels of epsilon2, although single channel conductances were identical between young and old mice. In addition, an increase in sensitivity to magnesium was observed for receptors from older animals. Results are consistent with the interpretation that the epsilon2 subunit is reduced in older mouse frontal cortex. A change in NMDA receptor subunit composition could influence memory processes during aging.

Slow NMDA-EPSCs at synapses critical for song development are not required for song learning in zebra finches

Birdsong, like human speech, is learned via auditory experience during a developmentally restricted sensitive period. Within projection neurons of two avian forebrain nuclei, NMDA receptor-mediated EPSCs (NMDA-EPSCs) become fast during song development, a transition posited to limit learning. To discover whether slow NMDA-EPSCs at these synapses are required for learning, we delayed song learning beyond its normal endpoint, post-hatch day (PHD) 65, by raising zebra finches in isolation from song tutors. At PHD45, before learning, isolation delayed NMDA-EPSC maturation, but only transiently. By PHD65, NMDA-EPSCs in isolates were fast and adult-like, yet isolates presented with tutors readily learned song. Thus song learning did not require slow NMDA-EPSCs at synapses critical for song development.

Rapid and transient learning-associated increase in NMDA NR1 subunit in the rat hippocampus

Several lines of evidence indicate that glutamate NMDA receptors are critically involved in long-term potentiation (LTP) and in certain forms of learning. It was previously demonstrated that memory formation of an inhibitory avoidance task in chick is specifically associated with an increase in the density of NMDA receptor in selected brain regions. Here we report on the effect of a one trial inhibitory avoidance training in rats, a hippocampal-dependent learning task, on the levels of different subunits of the glutamate NMDA receptor in synaptic plasma membranes (SPM) isolated from the hippocampus. Training rats on a one trial inhibitory avoidance task results in a rapid, transient and selective increase (+33%, p < 0.05) in NMDA NRI subunit expression in hippocampal SPM of rats sacrificed 30 min posttraining. No changes were observed at 0 or 120 min after training or in shocked animals in comparison to naive control rats. In addition, no training-associated increase in the levels of NMDA NR2A and NR2B or AMPA GluR 2/3 subunits was observed at any timepoint tested. In conclusion, the present findings support the hypothesis that alterations in expression of synaptic NMDA NR1 subunits in the hippocampus are specifically associated with memory formation of an inhibitory avoidance task and strongly suggest that hippocampal NMDA receptors are crucially involved in the neural mechanisms underlying certain forms of learning.

Roles of NMDA receptor activity and nitric oxide production in brain development

The concept that neural activity is important for brain maturation has focused much research interest on the developmental role of the NMDA receptor, a key mediator of experience-dependent synaptic plasticity. However, a mechanism able to link spatial and temporal parameters of synaptic activity during development emerged as a necessary condition to explain how axons segregate into a common brain region and make specific synapses on neuronal sub-populations. To comply with this developmental constraint, it was proposed that nitric oxide (NO), or other substances having similar chemical and biological characteristics, could act as short-lived, activity-dependent spatial signals, able to stabilize active synapses by diffusing through a local volume of tissue. The present article addresses this issue, by reviewing the experimental evidence for a correlated role of the activity of the NMDA receptor and the production of NO in key steps of neural development. Evidence for such a functional coupling emerges not only concerning synaptogenesis and formation of neural maps, for which it was originally proposed, but also for some earlier phases of neurogenesis, such as neural cell proliferation and migration. Regarding synaptogenesis and neural map formation in some cases, there is so far no conclusive experimental evidence for a coupled functional role of NMDA receptor activation and NO production. Some technical problems related to the use of inhibitors of NO formation and of gene knockout animals are discussed. It is also suggested that other substances, known to act as spatial signals in adult synaptic plasticity, could have a role in developmental plasticity. Concerning the crucial developmental phase of neuronal survival or elimination through programmed cell death, the well-documented survival role related to NMDA receptor activation also starts to find evidence for a concomitant requirement of downstream NO production. On the basis of the reviewed literature, some of the major controversial issues are addressed and, in some cases, suggestions for possible future experiments are proposed.

Linking Hebb's coincidence-detection to memory formation

The theoretical foundations of learning and memory were laid by Donald Hebb 50 years ago. Recent genetic experiments that enhanced coincidence-detection of the NMDA receptor (a molecular master-switch in implementing Hebb's rule) and that led to better learning and memory in adult animals have substantially validated Hebb's rule in memory formation in the brain.

Postnatal development of NR1, NR2A and NR2B immunoreactivity in the visual cortex of the rat

N-Methyl-D-aspartate receptors (NMDARs) are critically involved in some types of synaptic plasticity. The NMDAR subunits NR1, NR2A and NR2B are developmentally regulated, and it has been proposed that developmental changes in their expression may underlie developmental changes in cortical plasticity. Age-dependent change in cortical plasticity is most commonly measured by the monocular deprivation effect, which occurs during a critical period between P22 and P50 in the rat. Although the development of NMDAR subunits has been studied from birth through the fourth postnatal week, there is only meager information from older ages when visual plasticity ends. We hypothesized that there will be significant age-dependent change in expression of NR1, NR2A or NR2B between P22, when the cortex is plastic, and P90, when it is not. We applied specific antibodies recognizing NR1, NR2A and NR2B to the primary visual cortex at P14, P22, P30, P45 and P90. We found age-dependent changes in NR1-IR that were negatively correlated with changes in NR2A-IR; these subunits are not regulated in unison. In contrast, NR2A-IR and NR2B-IR were positively correlated. NR2A-IR and NR2B-IR both passed through a developmental minimum around P45, then recovered to approximately their P22 level. NR1-IR passed through a maximum at P45. There were no significant differences between P22 and P90. These results do not support the simple hypothesis that the loss of plasticity corresponds to a simple transition from juvenile levels of NMDAR subunit proteins to new adult levels. On the other hand, the results do confirm the hypothesis that there are significant changes in processing of NMDAR proteins during the time that plasticity is lost. How these changes of IR relate to synaptic transmission and plasticity needs to be clarified.

Declines in mRNA expression of different subunits may account for differential effects of aging on agonist and antagonist binding to the NMDA receptor

The purpose of the present study was to determine whether some of the age-related changes that occur in binding to the NMDA receptor complex can be accounted for by changes in subunit expression during the aging process. In situ hybridization for the NMDA subunits zeta1, epsilon1, and epsilon2, and receptor autoradiography, using the agonist glutamate and the competitive antagonist [(+/-)-2-carboxypiperazin-4-yl] propyl-1-phosphonic acid (CPP), were performed on sections from C57Bl/6 mice representing three different age groups (3, 10, and 30 months of age). There was a significant overall decrease between 3 and 30 month olds in the density of mRNA for the zeta1 subunit in the cortex and hippocampus, but only a few individual brain regions exhibited significant declines. The mRNA for the epsilon2 subunit was significantly decreased in a majority of cortical regions and in the dentate granule cells. Emulsion analysis indicated that the change in the density of epsilon2 subunit mRNA in the inner frontal cortex was primarily attributable to a decrease in the amount of messages per cell. Age-related changes in mRNA density of the epsilon2 subunit correlated with changes in NMDA-displaceable [(3)H]glutamate binding, and mRNA density changes in the zeta1 subunit showed a significant relationship with changes in [(3)H]CPP binding in the 30-month-old mice. These results suggest that changes during aging in the expression of different subunits of the NMDA receptor may account for the differential effects of aging on agonist versus antagonist binding to the NMDA binding site.

Enrichment induces structural changes and recovery from nonspatial memory deficits in CA1 NMDAR1-knockout mice

We produced CA1-specific NMDA receptor 1 subunit-knockout (CA1-KO) mice to determine the NMDA receptor dependence of nonspatial memory formation and of experience-induced structural plasticity in the CA1 region. CA1-KO mice were profoundly impaired in object recognition, olfactory discrimination and contextual fear memories. Surprisingly, these deficits could be rescued by enriching experience. Using stereological electron microscopy, we found that enrichment induced an increase of the synapse density in the CA1 region in knockouts as well as control littermates. Therefore, our data indicate that CA1 NMDA receptor activity is critical in hippocampus-dependent nonspatial memory, but is not essential for experience-induced synaptic structural changes.

Exon 5 and spermine regulate deactivation of NMDA receptor subtypes

Deactivation of N-methyl-D-aspartate (NMDA) channels after brief agonist exposure determines the duration of their synaptic activation during excitatory neurotransmission. We performed patch-clamp recordings of L-glutamate responses from human embryonic kidney tumoral cells (HEK293) expressing NR1 subunit variants lacking exon 5 together with the NR2B subunit. These responses had deactivation components that lasted several seconds. The presence of exon 5 or spermine greatly accelerated deactivation of L-glutamate responses through alterations in desensitization. These effects were also observed at positive holding potentials and in the presence of physiological Mg(2+). Thus NR1 splicing and polyamines may have profound effects on the kinetics of NMDA receptor-mediated synaptic transmission.

Genes for cognitive function: developments on the X

Developments in human genome research enabled the first steps toward a molecular understanding of cognitive function. That there are numerous genes on the X chromosome affecting intelligence at the lower end of the cognitive range is no longer in doubt. Naturally occurring mutations have so far led to the identification of seven genes accounting for a small proportion of familial nonspecific X-linked mental retardation. These new data indicate that normal expression of many more X-linked and autosomal genes contribute to cognitive function. The emerging knowledge implicating genes in intracellular signaling pathways provides the insight to identify as candidates other X-linked and autosomal genes regulating the normal development of cognitive function. Recent advances in unravelling the underlying molecular complexity have been spectacular but represent only the beginning, and new technologies will need to be introduced to complete the picture.

Critical periods during sensory development

Recent studies have made progress in characterizing the determinants of critical periods for experience-dependent plasticity. They highlight the role of neurotrophins, NMDA receptors and GABAergic inhibition. In particular, genetic manipulation of a single molecule, brain-derived neurotrophic factor (BDNF), has been shown to alter the timing of the critical period of plasticity in mouse visual cortex, establishing a causal relation between neurotrophin action, the development of visual function, and the duration of the critical period.

Ionotropic glutamate receptor expression in human spinal cord during first trimester development

Quantitative receptor autoradiography and immunoblotting were used to study the expression and distribution of AMPA, kainate and NMDA receptors in first trimester human spinal cord obtained from elective abortions ranging from 4 to 11.5 weeks of gestational age. Spinal cord tissue sections were processed for receptor autoradiography with the ligands [3H]AMPA, [3H]kainate and [3H]MK-801 and the optical density was measured separately in a dorsal region (alar plate) and ventral region (basal plate) of the autoradiographs. Binding sites for all three ligands were demonstrated already at 4-5.5 weeks of gestation and increased continuously during the first trimester both in the dorsal and ventral regions. [3H]AMPA binding to both high- and low-affinity sites increased from undetectable levels to about 35 and 400 fmol/mg tissue, respectively, during this period. A temporal difference in the distribution of [3H]AMPA binding sites was observed. The early homogeneous pattern of [3H]AMPA binding in both alar and basal plates had changed to a heterogeneous pattern at 11 weeks of gestation with the highest density of [3H]AMPA binding sites in the superficial layers of the immature dorsal horn. [3H]kainate and [3H]MK-801 binding sites were densely and homogeneously distributed already at 4 weeks, and steadily increased six- and two-fold, respectively, to about 100 fmol/mg tissue at 11.5 weeks of gestation. Immunoreactive bands corresponding to the NMDA receptor subunits NR1, NR2A, NR2B, NR2C and NR2D were demonstrated by immunoblotting at the earliest between 4.5 and 7 weeks and increasing concentrations were seen up to 11 weeks of gestation. These results suggest that AMPA, kainate and NMDA receptors are expressed in the human spinal cord early in embryogenesis.