Anesthesia induced retrograde amnesia is ameliorated by ephrinA5-IgG in mice: EphA receptor tyrosine kinases are involved in mammalian memory

Sevoflurane-induced amnesia is associated with inhibition of hippocampal cell ensemble activity after learning

General anesthesia could induce amnesia, however the mechanism remains unclear. We hypothesized that suppression of neuronal ensemble activity in the hippocampus by anesthesia during the post-learning period causes retrograde amnesia. To test this hypothesis, two experiments were conducted with sevoflurane anesthesia (2.5%, 30 min): a hippocampus-dependent memory task, the context pre-exposure facilitation effect (CPFE) procedure to measure memory function and in vivo calcium imaging to observe neural activity in hippocampal CA1 during context exploration and sevoflurane/home cage session. Sevoflurane treatment just after context pre-exposure session impaired the CPFE memory, suggesting sevoflurane induced retrograde amnesia. Calcium imaging showed sevoflurane treatment prevented neuronal activity in CA1. Further analysis of neuronal activity with non-negative matrix factorization, which extracts neural ensemble activity based on synchronous activity, showed that sevoflurane treatment reduced the reactivation of neuronal ensembles between during context exploration just before and one day after sevoflurane inhalation. These results suggest that sevoflurane treatment immediately after learning induces amnesia, resulting from suppression of reactivation of neuronal ensembles.

Crocins, the Bioactive Components of Crocus sativus L., Counteract the Disrupting Effects of Anesthetic Ketamine on Memory in Rats

Consistent experimental evidence suggests that anesthetic doses of the non-competitive N-methyl-d-aspartate (NMDA) receptor antagonist ketamine cause severe memory impairments in rodents. Crocins are among the various bioactive ingredients of the plant Crocus sativus L., and their implication in memory is well-documented. It has not yet been elucidated if crocins are able to attenuate the memory deficits produced by anesthetic ketamine. The present study was undertaken aiming to clarify this issue in the rat. For this aim, the object recognition, the object location and the habituation tests, reflecting non-spatial recognition memory, spatial recognition memory and associative memory, respectively, were utilized. A post-training challenge with crocins (15–30 mg/kg, intraperitoneally (i.p.), acutely) counteracted anesthetic ketamine (100 mg/kg, i.p.)-induced performance impairments in all the above-mentioned behavioral memory paradigms. The current findings suggest that crocins modulate anesthetic ketamine’s amnestic effects.

Gene-Specific DNA Methylation Linked to Postoperative Cognitive Dysfunction in Apolipoprotein E3 and E4 Mice

BACKGROUND

Geriatric surgical patients are at higher risk of developing postoperative neurocognitive disorders (NCD) than younger patients. The specific mechanisms underlying postoperative NCD remain unknown, but they have been linked to genetic risk factors, such as the presence of APOE4, compared to APOE3, and epigenetic modifications caused by exposure to anesthesia and surgery.

OBJECTIVE

To test the hypothesis that compared to E3 mice, E4 mice exhibit a more pronounced postoperative cognitive impairment associated with differential DNA methylation in brain regions linked to learning and memory.

METHODS

16-month-old humanized apolipoprotein-E targeted replacement mice bearing E3 or E4 were subjected to surgery (laparotomy) under general isoflurane anesthesia or sham. Postoperative behavioral testing and genome-wide DNA methylation were performed.

RESULTS

Exposure to surgery and anesthesia impaired cognition in aged E3, but not E4 mice, likely due to the already lower cognitive performance of E4 prior to surgery. Cognitive impairment in E3 mice was associated with hypermethylation of specific genes, including genes in the Ephrin pathway implicated in synaptic plasticity and learning in adults and has been linked to Alzheimer's disease. Other genes, such as the Scratch Family Transcriptional Repressor 2, were altered after surgery and anesthesia in both the E3 and E4 mice.

CONCLUSION

Our findings suggest that the neurocognitive and behavioral effects of surgery and anesthesia depend on baseline neurocognitive status and are associated with APOE isoform-dependent epigenetic modifications of specific genes and pathways involved in memory and learning.

Flumazenil but not bicuculline counteract the impairing effects of anesthetic ketamine on recognition memory in rats. Evidence for a functional interaction between the GABAA-benzodiazepine receptor and ketamine?

Experimental evidence indicates that anesthetic doses of the non-competitive NMDA receptor antagonist ketamine impair memory abilities in rodents. The mechanism by which anesthetic ketamine produces its adverse behavioural effects is not yet clarified. In this context, it has been proposed that the effects of anesthetic ketamine on memory might be attributed to its agonistic properties on the GABA type A receptor. The present study was designed to address this issue. Thus, we investigated the ability of the benzodiazepine receptor antagonist flumazenil (1, 3, 6 mg/kg, i.p.) and the GABA_A receptor antagonist bicuculline (0.5, 1.5, 3 mg/kg, i.p.) to counteract recognition memory deficits produced by anesthetic ketamine (100 mg/kg, i.p.) in rats. For this purpose, the novel object recognition task, a behavioural paradigm assessing recognition memory abilities in rodents was used. Compounds were coadministered 24 h before testing or retention. Pre (24 h before testing) or post-training (24 h before retention) administration of flumazenil (6 mg/kg, i.p.) counteracted anesthetic ketamine-induced performance deficits in the novel object recognition memory task. Conversely, bicuculline failed to attenuate the recognition memory deficits caused by anesthetic ketamine. Our findings propose a functional interaction between anesthetic ketamine and the GABA_A receptor allosteric modulator flumazenil on recognition memory.

EphA4 may contribute to microvessel remodeling in the hippocampal CA1 and CA3 areas in a mouse model of temporal lobe epilepsy

Unclustered and pre-clustered ephrin-A5-Fc have identical anti-epileptic effects in the dentate gyrus of hippocampus in a mouse model of temporal lobe epilepsy (TLE), and act through alleviating ephrin receptor A4 (EphA4)-mediated neurogenesis and angiogenesis. However, the effects of ephrin-A5-Fcs on EphA4 and angiogenesis in Cornu Ammonis (CA)1 and CA3 areas remain unclear. In the present study, male C57BL/6 mice underwent pilocarpine-induced TLE. The expression of EphA4 and ephrin-A5 proteins was analyzed by immunohistochemistry, and the mean density and diameter of platelet endothelial cell adhesion molecule-1-labeled microvessels in CA1 and CA3 were calculated in the absence or presence of two types of ephrin-A5-Fc intrahippocampal infusion. Microvessels perpendicular to the pyramidal cell layer decreased; however, microvessels that traversed the layer increased, and became distorted and fragmented. The mean densities and diameters of microvessels gradually increased and remained greater than those in the control group at 56 days post-status epilepticus (SE). The upregulation of EphA4 and ephrin-A5 proteins began at 7 days and was maintained until 28 days, subsequently decreasing slightly at 56 days post-SE. Blockade of EphA4 by unclustered-ephrin-A5-Fc effected a reduction in the mean density and mean diameter of microvessels in the CA1 and CA3 areas; conversely, activation of EphA4 by clustered-ephrin-A5-Fc induced an increase in these values. Ephrin-A5 ligand binding to EphA4 receptor may contribute to angiogenesis during epileptogenesis in the hippocampal CA1 and CA3 areas.

Eph Receptors, Ephrins, and Synaptic Function

Compelling new findings have revealed that receptor tyrosine kinases of the Eph family, along with their ephrin ligands, play an essential role in regulating the properties of developing mature excitatory synapses in the central nervous system. The cell surface localization of both the Eph receptors and the ephrins enables these proteins to signal bidirectionally at sites of cell-to-cell contact, such as synapses. Eph receptors and ephrins have indeed been implicated in multiple aspects of synaptic function, including clustering and modulating N-methyl-D-aspartate receptors, modifying the geometry of postsynaptic terminals, and influencing long-term synaptic plasticity and memory. In this review, we discuss how Eph receptors and ephrins are integrated into the molecular machinery that supports synaptic function.

The Ephrin-A5/EphA4 Interaction Modulates Neurogenesis and Angiogenesis by the p-Akt and p-ERK Pathways in a Mouse Model of TLE

Studies have shown that neurogenesis and angiogenesis do exist in temporal lobe epilepsy (TLE). The ephrin ligands and Eph receptors are the largest members of receptor tyrosine kinases, and their interaction via cell-cell contact participates in cell proliferation, differentiation, migration, and tissue remodeling. However, there is little information about the function of the ephrin-A5/EphA4 complex in TLE. In the current study, we found that ephrin-A5 was expressed in astrocytes, while EphA4 existed in endothelial cells in the hippocampus in a mouse model of TLE. Furthermore, the messenger RNA (mRNA) and protein levels of both ephrin-A5 and EphA4 and the binding capacity of ephrin-A5/EphA4 showed gradual increase in spatiotemporal course. When ephrin-A5-Fc was injected into the hippocampus at 3 days post-status epilepticus (SE) for 7 days, the spontaneous recurrent seizure (SRS) frequency and intensity of the mice attenuated in the following 2 weeks. Furthermore, doublecortin-positive neuronal progenitor cells were reduced in the subgranular zone, and the density of microvessels decreased in the hilus. The molecular mechanism was attributed to ephrin-A5-Fc-induced inhibition of phosphorylated ERK (p-ERK) and phosphorylated Akt (p-Akt), and also EphA4 and VEGF reduction. In summary, interaction between ephrin-A5 and EphA4 could mediate the ERK and Akt signaling pathways in pilocarpine-induced epilepsy, and intervention of the ephrin/Eph interaction may play an essential role in the suppression of newborn neuron generation, microvessel remodeling, and SRS in a mouse model of TLE. The ephrin-A5/EphA4 communication may provide a potential therapy for the treatment of TLE.

Role of synaptic plasticity and EphA5-ephrinA5 interaction within the ventromedial hypothalamus in response to recurrent hypoglycemia

Hypoglycemia stimulates counterregulatory hormone release to restore euglycemia. This protective response is diminished by recurrent hypoglycemia, limiting the benefits of intensive insulin treatment in patients with diabetes. We previously reported that EphA5 receptor-ephrinA5 interactions within the ventromedial hypothalamus (VMH) influence counterregulatory hormone responses during acute hypoglycemia in nondiabetic rats. In this study, we examined whether recurrent hypoglycemia alters the capacity of the ephrinA5 ligand to activate VMH EphA5 receptors, and if so, whether these changes could contribute to pathogenesis of defective glucose counterregulation in response to a standard hypoglycemic stimulus. The expression of ephrinA5, but not EphA5 receptors within the VMH, was reduced by antecedent recurrent hypoglycemia. In addition, the number of synaptic connections was increased and astroglial synaptic coverage was reduced. Activation of VMH EphA5 receptors via targeted microinjection of ephrinA5-Fc before a hyperinsulinemic hypoglycemic clamp study caused a reduction in the glucose infusion rate in nondiabetic rats exposed to recurrent hypoglycemia. The increase in the counterregulatory response to insulin-induced hypoglycemia was associated with a 150% increase in glucagon release (P < 0.001). These data suggest that changes in ephrinA5/EphA5 interactions and synaptic plasticity within the VMH, a key glucose-sensing region in the brain, may contribute to the impairment in glucagon secretion and counterregulatory responses caused by recurrent hypoglycemia.

EphA5-EphrinA5 interactions within the ventromedial hypothalamus influence counterregulatory hormone release and local glutamine/glutamate balance during hypoglycemia

Activation of β-cell EphA5 receptors by its ligand ephrinA5 from adjacent β-cells has been reported to decrease insulin secretion during hypoglycemia. Given the similarities between islet and ventromedial hypothalamus (VMH) glucose sensing, we tested the hypothesis that the EphA5/ephrinA5 system might function within the VMH during hypoglycemia to stimulate counterregulatory hormone release as well. Counterregulatory responses and glutamine/glutamate concentrations in the VMH were assessed during a hyperinsulinemic-hypoglycemic glucose clamp study in chronically catheterized awake male Sprague-Dawley rats that received an acute VMH microinjection of ephrinA5-Fc, chronic VMH knockdown, or overexpression of ephrinA5 using an adenoassociated viral construct. Local stimulation of VMH EphA5 receptors by ephrinA5-Fc or ephrinA5 overexpression increased, whereas knockdown of VMH ephrinA5 reduced counterregulatory responses during hypoglycemia. Overexpression of VMH ephrinA5 transiently increased local glutamate concentrations, whereas ephrinA5 knockdown produced profound suppression of VMH interstitial fluid glutamine concentrations in the basal state and during hypoglycemia. Changes in ephrinA5/EphA5 interactions within the VMH, a key brain glucose-sensing region, act in concert with islets to restore glucose homeostasis during acute hypoglycemia, and its effect on counterregulation may be mediated by changes in glutamate/glutamine cycling.

In vivo imaging and quantitative analysis of changes in axon length using transgenic zebrafish embryos

We describe an imaging procedure to measure axon length in zebrafish embryos in vivo. Automated fluorescent image acquisition was performed with the ImageXpress Micro high content screening reader and further analysis of axon lengths was performed on archived images using AcuityXpress software. We utilized the Neurite Outgrowth Application module with a customized protocol (journal) to measure the axons. Since higher doses of ethanol (2-2.5%, v/v) have been shown to deform motor neurons and axons during development, here we used ethanol to treat transgenic [hb9:GFP (green fluorescent protein)] zebrafish embryos at 28 hpf (hours post-fertilization). These embryos express GFP in the motor neurons and their axons. Embryos after ethanol treatment were arrayed in 384-well plates for automated fluorescent image acquisition in vivo. Average axon lengths of high dose ethanol-treated embryos were significantly lower than the control. Another experiment showed that there was no significant difference in the axon lengths between the embryos grown for 24h at 22°C and 28.5°C. These test experiments demonstrate that using axon development as an end-point, compound screening can be performed in a time-efficient manner.

Heat shock protein 72 overexpression prevents early postoperative memory decline after orthopedic surgery under general anesthesia in mice

BACKGROUND

Problems with learning and memory are common after surgery in the elderly and are associated with high morbidity. Heat shock protein 72 (Hsp72) confers neuroprotection against acute neurologic injury. We hypothesized that overexpression of Hsp72 would prevent the development of postoperative memory loss.

METHODS

C57BL/6 wild-type and Hsp72 overexpressing transgenic mice were randomly allocated to the following: control, isoflurane anesthesia alone, or tibial fracture during isoflurane anesthesia. Animals were trained 24 h before surgery using a fear conditioning protocol and assessed in their training environment and in a novel context on posttreatment days 1, 3, and 7. Microglial activation was assessed by immunostaining.

RESULTS

Adult male C57BL/6 wild-type mice exhibited reduced memory evidenced by a decreased percentage freezing time on days 1 and 3 after anesthesia alone (58.8 ± 5, 46.5 ± 5 mean ± SEM) and after surgery (53.4 ± 6, 44.1 ± 7), compared with controls (78.8 ± 5, 63.4 ± 6; P < 0.05 and P < 0.001, respectively). Hsp72 mice showed no difference by treatment on any day. Similarly, nonhippocampal-dependent memory was significantly impaired on days 1 and 3 after surgery and day 3 after anesthesia. The genotype effect was significant on days 1 and 7. CD68-immunopositive activated microglia in the hippocampus varied modestly with subregion and time; on day 7, there was a significant treatment effect with no genotype effect, with more activated microglia after surgery in all regions.

CONCLUSION

Hsp72 overexpression is associated with prevention of postoperative hippocampal-dependent and -independent memory deficit induced by anesthesia and/or surgery. Memory deficit is not correlated with numbers of activated hippocampal microglia.

Hyperphosphorylated tau in the brains of mice and monkeys with long-term administration of ketamine

Ketamine, a non-competitive antagonist at the glutamatergic N-methyl-d-aspartate (NMDA) receptor, might impair memory function of the brain. Loss of memory is also a characteristic of aging and Alzheimer's disease. Hyperphosphorylation of tau is an early event in the aging process and Alzheimer's disease. Therefore, we aimed to find out whether long-term ketmaine administration is related to hyperphosphorylation of tau or not in the brains of mice and monkeys. Results showed that after 6 months' administration of ketamine, in the prefrontal and entorhinal cortical sections of mouse and monkey brains, there were significant increases of positive sites for the hyperphosphorylated tau protein as compared to the control animals receiving no ketamine administration. Furthermore, about 15% of hyperphosphorylated tau positive cells were also positively labeled by terminal dUTP nick end labeling (TUNEL) indicating there might be a relationship between hyperphosphorylation of tau and apoptosis. Therefore, the long-term ketamine toxicity might involve neurodegenerative process similar to that of aging and/or Alzheimer's disease.

The genetics of episodic memory

INTRODUCTION

Studies suggest that there is a considerable genetic contribution to individual episodic memory performance. Identifying genes which impact recollection may further elucidate an emerging biology and pave the way towards novel cognitive interventions. To date, several candidate genes have been explored and a few seem to have modest but measurable effects.

METHODS

Here we review the biology of memory with particular focus on episodic memory, critically appraise the published evidence supporting the role of several candidate genes, and make suggestions for future pathways of research.

RESULTS

We found moderate evidence for several candidate genes implicated in episodic memory formation, with converging lines of neurobiologic evidence especially strong for only a select few. Perhaps unexpectedly, little work has been done on other aspects of memory, including the semantic and autobiographical systems.

CONCLUSIONS

Larger studies utilizing more elaborate methodologies to measure the spectrum of episodic memory are required to move the field forward.

Localization of EphA4 in axon terminals and dendritic spines of adult rat hippocampus

Eph receptors and their ephrin ligands assume various roles during central nervous system development. Several of these proteins are also expressed in the mature brain, and notably in the hippocampus, where EphA4 and ephrins have been shown to influence dendritic spine morphology and long-term potentiation (LTP). To examine the cellular and subcellular localization of EphA4 in adult rat ventral hippocampus, we used light and electron microscopic immunocytochemistry with a specific polyclonal antibody against EphA4. After immunoperoxidase labeling, EphA4 immunoreactivity was found to be enriched in the neuropil layers of CA1, CA3, and dentate gyrus. In all examined layers of these regions, myelinated axons, small astrocytic leaflets, unmyelinated axons, dendritic spines, and axon terminals were immunolabeled in increasing order of frequency. Neuronal cell bodies and dendritic branches were immunonegative. EphA4-labeled dendritic spines and axon terminals corresponded to 9-19% and 25-40% of the total number of spines and axon terminals, respectively. Most labeled spines were innervated by unlabeled terminals, but synaptic contacts between two labeled elements were seen. The vast majority of synaptic junctions made by labeled elements was asymmetrical and displayed features of excitatory synapses. Immunogold labeling of EphA4 was located mostly on the plasma membrane of axons, dendritic spines, and axon terminals, supporting its availability for surface interactions with ephrins. The dual preferential labeling of EphA4 on pre- or postsynaptic specializations of excitatory synapses in adult rat hippocampus is consistent with roles for this receptor in synaptic plasticity and LTP.

Bidirectional ephrin/Eph signaling in synaptic functions

Eph receptors, the largest family of receptor tyrosine kinases, and their membrane bound ligands, the ephrins, are involved in multiple developmental and adult processes within and outside of the nervous system. Bi-directional signaling from both the receptor and the ligand is initiated by ephrin-Eph binding upon cell-cell contact, and involves interactions with distinct subsets of downstream signaling molecules related to specific functions. In the CNS, Ephs and ephrins act as attractive/repulsive, migratory and cell adhesive cues during development and participate in synaptic functions in adult animals. In this review, we will focus on recent findings highlighting the functions of ephrin/Eph signaling in dendritic spine morphogenesis, synapse formation and synaptic plasticity.

Ketamine affects memory consolidation: Differential effects in T-maze and passive avoidance paradigms in mice

The effects of ketamine, an N-methyl-D-aspartate (NMDA) antagonist, on memory in animals have been limited to the sub-anesthetic dose given prior to training in previous studies. We evaluated the effects of post-training anesthetic doses of ketamine to selectively manipulate memory consolidation, and the effect of pre-retention sub-anesthetic doses of ketamine on memory retrieval in passive avoidance and T-maze tasks in mice. Repeated post-training anesthetic doses of ketamine impaired the consolidation of memory in the T-maze but not in passive avoidance paradigms. This impairment was not permanent but diminished 1-2 days after ketamine withdrawal. Sub-anesthetic post-training doses of ketamine (5 mg/kg) had no effect on memory consolidation, and larger doses (10, 20 and 50 mg/kg) did not influence the retrieval of memory in the T-maze. The data suggest that repeated anesthetic doses of ketamine block NMDA receptors and affect memory consolidation. Moreover, NMDA mechanisms antagonized by ketamine appear to be selectively involved in spatial (T-maze) memory mechanisms but may not be necessary for non-spatial (passive avoidance) memory consolidation.

Stage 2 of the Wellcome Trust UK-Irish bipolar affective disorder sibling-pair genome screen: evidence for linkage on chromosomes 6q16-q21, 4q12-q21, 9p21, 10p14-p12 and 18q22

Bipolar affective disorder (BPAD) is a common psychiatric disorder with complex genetic aetiology. We have undertaken a genome-wide scan in one of the largest samples of bipolar affected sibling pairs (ASPs) using a two-stage approach combining sample splitting and marker grid tightening. In this second stage analysis, we have examined 17 regions that achieved a nominally significant maximum likelihood LOD score (MLS) threshold of 0.74 (or 1.18 for the X-chromosome) in stage one. The second stage has added 135 ASP families to bring the total stage 2 sample to 395 ASPs. In total, 494 microsatellite markers have been used to screen the human genome at a density of 10 cM in the first stage sample (260 ASPs) and 5 cM in the second stage. Under the broad diagnostic model, two markers gave LOD scores exceeding 3 with two-point analysis: D4S392 (LOD=3.30) and D10S197 (LOD=3.18). Multipoint analysis demonstrated suggestive evidence of linkage between BPAD and chromosomal regions 6q16-q21 (MLS=2.61) and 4q12-q21 (MLS=2.38). 6q16-q21 is of particular interest because our data, together with those from two recent genome scans, make this the best supported linkage region in BPAD. Further, our data show evidence of a gender effect at this locus with increased sharing predominantly within the male-male pairs. Our scan also provides support for linkage (MLS> or =1.5) at several other regions that have been implicated in meta-analyses of bipolar disorder and/or schizophrenia including 9p21, 10p14-p12 and 18q22.

Induction of ephrin-B1 and EphB receptors during denervation-induced plasticity in the adult mouse hippocampus

Abstract It has been widely demonstrated that Eph receptors and their ephrin ligands play multiple pivotal roles in the development of the nervous system. However, less is known about their roles in the adult brain. Here we reported the expression of ephrin-B1 and its cognate EphB receptors in the adult mouse hippocampus at 3, 7, 15, 30 and 60 days after transections of the entorhinal afferents. In situ hybridization and immunohistochemistry showed the time-dependent up-regulation of ephrin-B1 in the denervated areas of the hippocampus, which initiated at 3 days postlesion (dpl), reached maximal levels at 7-15 dpl, remained slightly elevated at 30 dpl and recovered to normal levels by 60 dpl. Double labeling of ephrin-B1 and glial fibrillary acidic protein revealed that ephrin-B1-expressing cells in the denervated areas were reactive astrocytes. Furthermore, a ligand-binding assay using ephrin-B1/Fc chimera protein also displayed the up-regulation of EphB receptors in the denervated areas of the hippocampus in a similar manner to that of ephrin-B1. Within the first week postlesion, the EphB receptors were expressed by reactive astrocytes. After 7 dpl, however, EphB receptors were expressed not only by reactive astrocytes but also first by sprouting axons and later by regrowing dendrites. These results suggest that the ephrin-B1/EphB system may participate in the lesion-induced plasticity processes in the adult mouse hippocampus.

Functions of ephrin/Eph interactions in the development of the nervous system: emphasis on the hippocampal system

Ephrins and their Eph receptors are membrane-anchored proteins that have key roles in the development of the Central Nervous System. The main characteristics of ephrin/Eph interactions are that their effect is mediated by cell-to-cell contacts and that they can propagate bidirectional signals downstream of the ligand-receptor complex. These characteristics make ephrins and Eph receptors critical cues in the regulation of migrating cells or axons, and in the establishment of tissue patterns and topographic maps in distinct regions of the developing brain. In addition, ephrins and Eph receptors regulate synapse formation and plasticity. These roles would be promoted by complementary gradual expression of receptors and ligands in the neurons involved. Although, historically, ephrins and Eph receptors have been considered as repulsion signals through barriers or gradients, new evidence indicates that they may be both inhibitory and permissive/active cues depending on expression levels. The expression of distinct ligands and receptors in the developing and mature hippocampus suggests that these proteins are involved in distinct processes during the development and maturation of the hippocampal region. In fact, recent studies have shown that ephrin/Eph signaling participates in the formation of the layer-specific patterns of hippocampal afferents, in synaptogenesis and in plasticity. Therefore, ephrin/Eph interactions should be considered a crucial system in the development and maturation of the brain regions, including the hippocampus.

Eph receptors in the adult brain

The Eph receptors are a large family of receptor tyrosine kinases with important roles in the establishment of neuronal and vascular networks during embryonic development. The functions of Eph receptors in the adult brain have only recently been investigated, and the results are forcing us to amend the conventional view that these molecules function predominantly in a developmental context. This review summarizes this rapidly expanding new area of research, which has shown that the Eph receptors regulate the structure and physiological function of excitatory synapses through multiple mechanisms, and might thus play a significant role in higher brain functions.

The effect of repeated isoflurane anesthesia on spatial and psychomotor performance in young and aged mice

Exposure to general anesthesia may contribute to postoperative cognitive impairment in elderly patients, but the relationship remains poorly understood. We investigated whether aged mice, 18-19 mo, are more susceptible to postanesthetic cognitive impairment than young mice, 3-4 mo, using spatial memory (Barnes maze) and psychomotor (rotarod) tasks. Initially we studied the effect of a single anesthetic episode on asymptotic maze performance. We then tested whether repeated anesthesia would impair spatial memory and psychomotor performance to a greater extent in aged mice. Mice were anesthetized with isoflurane (1.4% atm) for 30 min; controls received 90% oxygen. Anesthesia, administered during the asymptotic period of maze learning, did not impair performance tested the following day (P > 0.05). Repeated anesthesia, 2-3 h after each session, did not impair overall maze or rotarod performance in young or aged mice (P > 0.05). Spatial learning appeared to be facilitated by anesthesia, F(1,204) = 7.97, P < 0.01 for pooled results. Asymptotic performance-when learning had stabilized-remained unimpaired in both the maze and rotarod tasks. These results suggest that an age-related risk of anesthetic-induced impairment appears to be limited to acquisition of a novel motor skill and that anesthesia alone does not lead to prolonged cognitive impairments in aged mice.

IMPLICATIONS

This study demonstrates that repeated isoflurane general anesthesia impaired psychomotor performance in aged mice during the initial learning period; however, spatial learning improved and, overall, spatial memory and psychomotor performance were unimpaired. Thus, general anesthesia alone does not appear to result in prolonged cognitive deficits in aged mice.

Neurochemical and behavioral deficits consequent to expression of a dominant negative EphA5 receptor

The Eph family tyrosine kinase receptors and their ligands have been linked to axon guidance and topographic mapping of the developing central nervous system. More specifically, the EphA5 receptor has been shown to play a role in development of hippocamposeptal, retinotectal and thalamocortical projections. Recently, a line of transgenic mice was developed which expresses a truncated EphA5 receptor lacking a functional tyrosine kinase domain. In a previous study, axonal tracing revealed that medial hippocampal axons in this strain projected laterally and ventrally away from their normal target area. In the current study, both transgenic and wild-type controls were evaluated in unconditioned (rotorod and locomotor activity) and conditioned (water maze and active avoidance) behavior tasks which tested hippocampal and striatal functioning. Compared to controls, the transgenic strain did not show differences in rotorod motor activity but did show a transient deficit in spatial navigation ability and a consistent impairment in active avoidance. The dominant-negative mutant receptor also resulted in a decrease in striatal dopamine and serotonin concentrations with no change in hippocampal monoamines. Collectively, these data suggest that animals expressing a truncated EphA5 receptor show deficits related to striatal functioning.

Tyrosine kinases, synaptic plasticity and memory: insights from vertebrates and invertebrates

Tyrosine kinases were first characterized in terms of their function during development. Over the past decade, it has become clear that tyrosine phosphorylation also plays an important role in the adult mammalian nervous system. This article reviews three different families of tyrosine kinase signaling cascades: the Trk receptor tyrosine kinases, the Src family of non-receptor tyrosine kinases and the Eph receptor tyrosine kinases. Each of these cascades has been implicated in both adult synaptic plasticity and memory formation. Evidence from invertebrate systems also demonstrates a role for tyrosine kinase signaling in the induction of long-term memory, suggesting that molecular mechanisms of memory formation are conserved across species.

Control of hippocampal dendritic spine morphology through ephrin-A3/EphA4 signaling

Communication between glial cells and neurons is emerging as a critical parameter of synaptic function. However, the molecular mechanisms underlying the ability of glial cells to modify synaptic structure and physiology are poorly understood. Here we describe a repulsive interaction that regulates postsynaptic morphology through the EphA4 receptor tyrosine kinase and its ligand ephrin-A3. EphA4 is enriched on dendritic spines of pyramidal neurons in the adult mouse hippocampus, and ephrin-A3 is localized on astrocytic processes that envelop spines. Activation of EphA4 by ephrin-A3 was found to induce spine retraction, whereas inhibiting ephrin/EphA4 interactions distorted spine shape and organization in hippocampal slices. Furthermore, spine irregularities in pyramidal neurons from EphA4 knockout mice and in slices transfected with kinase-inactive EphA4 indicated that ephrin/EphA4 signaling is critical for spine morphology. Thus, our data support a model in which transient interactions between the ephrin-A3 ligand and the EphA4 receptor regulate the structure of excitatory synaptic connections through neuroglial cross-talk.

Can Eph receptors stimulate the mind?

The Eph receptors are multitalented tyrosine kinases capable of performing many tasks. The receptors together with their ligands--the ephrins--are well known to play a critical role in the initial assembly of neuronal circuits in the embryo. However, the recently discovered function of these receptors in the adult brain is now receiving significant acclaim. Three new articles show that the Eph receptors continue to be important in modifying the strength of existing neuronal connections (synapses). They do so in close association with at least one family of ion channels, the NMDA receptors.

Semiquantitative expression analysis of ephrine-receptor tyrosine kinase mRNA's in a rat model of traumatic brain injury

The molecular mechanisms involved in recovery of function of the central nervous system (CNS) after injury to the brain are incompletely understood. Here the expression of ephrine (Eph) kinases following traumatic brain injury (subdural haematoma) was analysed in order to find out whether these developmentally regulated genes may be involved in tissue remodelling after brain damage. mRNA was isolated from ipsilateral cortices 7, 18, and 28 days after surgery and semiquantitative reverse transcription-polymerase chain reaction was performed. Most Eph kinases did not show significant regulation at gene expression level during the time course of recovery from acute brain injury but there is some evidence that mRNA of EphB1 might be slightly upregulated.

Eph receptors and neural plasticity

Eph receptor tyrosine kinases are largely known for their involvement in brain development but, as some of these receptor tyrosine kinases are also expressed in adults, their possible role in the mature nervous system has begun to be explored. Evidence for the involvement of Eph receptors in synaptic plasticity, learning and memory is only emerging and needs corroboration. However, it is likely that the actions of Eph kinases in the adult brain will attract significant attention and become a fertile research area, as occurred in the case of the neurotrophins.

Protein targeting: altering receptor kinase function in the brain

Gene targeting has proved to be one of the most powerful techniques with which one can investigate molecular mechanisms that underlie complex phenomena such as learning and memory. Despite its popularity, however, concerns have been raised about this technique and alternative approaches have been sought. One such approach is protein targeting, which is based on the application of immunoadhesins, genetically engineered fusion proteins that exhibit functionally relevant target specificity. These immunoadhesins modulate the activity of not only a single receptor but of all receptors with homologous binding sites, which thereby eliminates the possibility of compensation by sister receptors. Furthermore, immunoadhesins can be used not only to impair but also to improve receptor function in the brain. Initial studies using immunoadhesins suggest that protein targeting might be a useful approach for analyzing the molecular mechanisms of brain function and behavior.