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Biomedical subjects

E R Kandel

Publications and source records attributed to E R Kandel.

At least 91 records · Page 5Linked to original sources
PubMed1996

Mice lacking the gene encoding tissue-type plasminogen activator show a selective interference with late-phase long-term potentiation in both Schaffer collateral and mossy fiber pathways.

The gene encoding tissue-type plasminogen activator (t-PA) is an immediate response gene, downstream from CREB-1 and other constitutively expressed transcription factors, which is induced in the hippocampus during the late phase of long-term potentiation (L-LTP). Mice in which the t-PA gene has been ablated (t-PA-/-) showed no gross anatomical, electrophysiological, sensory, or motor abnormalities but manifest a selective reduction in L-LTP in hippocampal slices in both the Schaffer collateral-CA1 and mossy fiber-CA3 pathways. t-PA-/- mice also exhibit reduced potentiation by cAMP analogs and D1/D5 agonists. By contrast, hippocampal-dependent learning and memory were not affected in these mice, whereas performance was impaired on two-way active avoidance, a striatum-dependent task. These results provide genetic evidence that t-PA is a downstream effector gene important for L-LTP and show that modest impairment of L-LTP in CA1 and CA3 does not result in hippocampus-dependent behavioral phenotypes.

Animals↗
PubMed1996

A macromolecular synthesis-dependent late phase of long-term potentiation requiring cAMP in the medial perforant pathway of rat hippocampal slices.

Memory storage consists of a short-term phase that is independent of new protein synthesis and a long-term phase that requires the synthesis of new proteins and RNA. A cellular representation of these two phases has been demonstrated recently for long-term potentiation (LTP) in both the Schaffer collateral and the mossy fibers of the hippocampus, a structure widely thought to contribute to memory consolidation. By contrast, much less information is available about the medial perforant pathway (MPP), one of the major inputs to the hippocampus. We found that both a short-lasting and a long-lasting potentiation (L-LTP) can be induced in the MPP of rat hippocampal slices by applying repeated tetanization in reduced levels of magnesium. This potentiation was dependent on the activation of NMDA receptors. The early, transient phase of LTP in the MPP did not require either protein or RNA synthesis, and it was independent of protein kinase A activation. By contrast, L-LTP required the synthesis of proteins and RNA, and was selectively blocked by inhibitors of cAMP-dependent protein kinase (PKA). Forskolin, an adenylate cyclase activator, also induced a L-LTP that was attenuated by inhibition of transcription. Our results demonstrate that, like LTP in the Schaffer collateral and mossy fiber pathways, MPP LTP also consists of a late phase that is dependent on protein and RNA synthesis and PKA activity. Thus, cAMP-mediated transcription appears to be a common mechanism for the late form of LTP in all three pathways within the hippocampus.

Animals↗
PubMed1996

Reconstitution of the hippocampal mossy fiber and associational-commissural pathways in a novel dissociated cell culture system.

Synapses of the hippocampal mossy fiber pathway exhibit several characteristic features, including a unique form of long-term potentiation that does not require activation of the N-methyl-D-aspartate receptor by glutamate, a complex postsynaptic architecture, and sprouting in response to seizures. However, these connections have proven difficult to study in hippocampal slices because of their relative paucity (<0.4%) compared to commissural-collateral synapses. To overcome this problem, we have developed a novel dissociated cell culture system in which we have enriched mossy fiber synapses by increasing the ratio of granule-to-pyramidal cells. As in vivo, mossy fiber connections are composed of large dynorphin A-positive varicosities contacting complex spines (but without a restricted localization). The elementary synaptic connections are glutamatergic, inhibited by dynorphin A, and exhibit N-methyl-D-aspartate-independent long-term potentiation. Thus, the simplicity and experimental accessibility of this enriched in vitro mossy fiber pathway provides a new perspective for studying nonassociative plasticity in the mammalian central nervous system.

Animals↗
PubMed1996

A presynaptic locus for long-term potentiation of elementary synaptic transmission at mossy fiber synapses in culture.

The complex circuitry of the CA3 region and the abundance of collateral connections has made it difficult to study the mossy fiber pathway in hippocampal slices and therefore to establish the site of expression of long-term potentiation at these synapses. Using a novel cell culture system, we have produced long-term potentiation of the elementary synaptic connections on single CA3 pyramidal neurons following tetanic stimulation of individual dentate gyrus granule cells. As is the case for the hippocampal slice, this potentiation was independent of N-methyl-D-aspartate receptor activation, was simulated by application of forskolin, and its induction did not require any modulatory input. The increase in synaptic strength was accompanied by a reduction in the number of failures of transmission and by an increase in the coefficient of variation of the responses and was prevented by presynaptic injection of an inhibitor of protein kinase A. These findings show that mossy fiber long-term potentiation has a presynaptic locus and that its expression is dependent on protein kinase A.

Animals↗
PubMed1996

Impaired hippocampal plasticity in mice lacking the Cbeta1 catalytic subunit of cAMP-dependent protein kinase.

Neural pathways within the hippocampus undergo use-dependent changes in synaptic efficacy, and these changes are mediated by a number of signaling mechanisms, including cAMP-dependent protein kinase (PKA). The PKA holoenzyme is composed of regulatory and catalytic (C) subunits, both of which exist as multiple isoforms. There are two C subunit genes in mice, Calpha and Cbeta, and the Cbeta gene gives rise to several splice variants that are specifically expressed in discrete regions of the brain. We have used homologous recombination in embryonic stem cells to introduce an inactivating mutation into the mouse Cbeta gene, specifically targeting the Cbeta1-subunit isoform. Homozygous mutants showed normal viability and no obvious pathological defects, despite a complete lack of Cbeta1. The mice were analyzed in electrophysiological paradigms to test the role of this isoform in long-term modulation of synaptic transmission in the Schaffer collateral-CA1 pathway of the hippocampus. A high-frequency stimulus produced potentiation in both wild-type and Cbeta1-/- mice, but the mutants were unable to maintain the potentiated response, resulting in a late phase of long-term potentiation that was only 30% of controls. Paired pulse facilitation was unaffected in the mutant mice. Low-frequency stimulation produced long-term depression and depotentiation in wild-type mice but failed to produce lasting synaptic depression in the Cbeta1 -/- mutants. These data provide direct genetic evidence that PKA, and more specifically the Cbeta1 isoform, is required for long-term depression and depotentiation, as well as the late phase of long-term potentiation in the Schaffer collateral-CA1 pathway.

Animals↗
PubMed1996

Presynaptic facilitation revisited: state and time dependence.

The mechanisms underlying short-term presynaptic facilitation, the enhancement of transmitter release from sensory neurons in Aplysia, induced by serotonin (5-HT), can be divided into two categories: (1) changes in ionic conductances leading to spike broadening and enhancement of Ca2+ influx; and (2) actions on the machinery for transmitter release that are independent of spike broadening and the resulting increases in Ca2+ influx. Spike broadening and the associated enhancement of excitability are induced by the modulation of K+ conductances in the sensory neuron. The cellular mechanisms that contribute to the enhancement of release that is independent of spike broadening are not known and may involve vesicle mobilization or other steps in exocytotic release. These two facilitatory actions of 5-HT are mediated by at least two second-messenger-activated protein kinase systems, protein kinase A (PKA) and protein kinase C (PKC). These two second-messenger cascades overlap in their contributions to synaptic facilitation. However, their relative contributions to enhancement of transmitter release are not simply synergistic but are state- and time-dependent. The state dependence is a reflection of the synapse's previous history of activity. When the synapse is rested (and not depressed), a brief pulse of 5-HT (lasting from 10 sec to 5 min) produces its actions primarily through PKA via both spike broadening-dependent and -independent mechanisms. The broadening primarily involves the modulation of a voltage-dependent K+ current, IKV, with a small contribution by a voltage-independent K+ current, IKS. By contrast, the enhancement of excitability is mediated primarily by the modulation of IKS. As the synapse becomes depressed with repeated activity, the contribution of PKC becomes progressively more important. As is the case with PKA, PKC produces its action both by broadening the spike via modulation of IKV and by a spike broadening-independent mechanism. In addition to being state-dependent, the mechanisms of facilitation are time-dependent. There are differences in the response to 5-HT when it is given briefly to produce short-term facilitation or when the exposure is prolonged. When exposure is brief (< or = 5 min), PKA dominates. When exposure is prolonged (10-20 min), PKC becomes dominant as it is with depressed synapses. Thus, synaptic plasticity appears to be expressed in several overlapping time domains, and the transition between very short-term facilitation and various intermediate duration phases seems to involve interactive processes between the kinases.

Animals↗
PubMed1996

Modulation of both the early and the late phase of mossy fiber LTP by the activation of beta-adrenergic receptors.

cAMP and the cAMP-dependent protein kinase (PKA) are required in the mossy fiber pathway for both the early and the late phase of long-term potentiation (LTP). Since the CA3 region, which is the target of the mossy fibers, receives extensive noradrenergic innervation, we examined the role of beta-adrenergic receptors in mossy fiber LTP. We found that we could induce an early phase of LTP by pairing isoproterenol, a beta-adrenergic receptor agonist, with a weak train, subthreshold for LTP. This LTP was specific to the mossy fiber pathway, was blocked by inhibitors of PKA, and occluded paired-pulse facilitation, suggesting that isoproterenol was acting presynaptically. When isoproterenol was paired with a train that induced only the early phase of LTP, it gave rise to a protein synthesis-dependent late phase. Consistent with these findings, beta-adrenergic antagonists blocked both the late and the early phase of LTP induced by mossy fiber stimulation. Thus, beta-adrenergic receptor modulation is important for both phases of mossy fiber LTP.

Animals↗
PubMed1996

Recombinant BDNF rescues deficits in basal synaptic transmission and hippocampal LTP in BDNF knockout mice.

Brain-derived neurotrophic factor (BDNF) is expressed at high levels in hippocampal neurons, and its expression is modulated by neural activity. Knockout mice can be used to study the roles of molecules like BDNF in synaptic plasticity with more molecular specificity than is possible using pharmacological approaches. Because in conventional knockouts the disrupted gene product is absent in all tissues throughout the life of the animal, developmental effects may complicate the interpretation of deficits in the adult. Rescue experiments can help to distinguish between developmental and acute requirements for the missing gene product. We here demonstrate that treatment of hippocampal slices from BDNF knockout mice with recombinant BDNF completely reverses deficits in long-term potentiation and significantly improves deficits in basal synaptic transmission at the Schaffer collateral-CA1 synapse. Thus, BDNF has an acute role in hippocampal synaptic function.

Animals↗
PubMed1996

Nitric oxide acts as a retrograde messenger during long-term potentiation in cultured hippocampal neurons.

We examined long-term potentiation (LTP) at synapses between hippocampal neurons in dissociated cell culture following presynaptic, postsynaptic, or extracellular application of a nitric oxide (NO) scavenger, an inhibitor of NO synthase, and a membrane-impermeant NO donor that releases NO only upon photolysis with UV light. Our results indicate that NO is produced in the postsynaptic neuron, travels through the extracellular space, and acts directly in the presynaptic neuron to produce long-term potentiation, supporting the hypothesis that NO acts as a retrograde messenger during LTP.

Animals↗
PubMed1995

A genetic test of the effects of mutations in PKA on mossy fiber LTP and its relation to spatial and contextual learning.

Using a genetic approach, we assessed the effects of mutations in protein kinase A (PKA) on long-term potentiation (LTP) in the mossy fiber pathway and its relationship to spatial and contextual learning. Ablation by gene targeting of the C beta 1 or the RI beta isoform of PKA produces a selective defect in mossy fiber LTP, providing genetic evidence for the role of these isoforms in the mossy fiber pathway. Despite the elimination of mossy fiber LTP, the behavioral responses to novelty, spatial learning, and conditioning to context are unaffected. Thus, contrary to current theories about hippocampal function, mossy fiber LTP does not appear to be required for spatial or contextual learning. In the absence of mossy fiber LTP, adequate spatial and contextual information might reach the CA1 region via other pathways from the entorhinal cortex.

Animals↗
PubMed1995

Aplysia CREB2 represses long-term facilitation: relief of repression converts transient facilitation into long-term functional and structural change.

The switch from short- to long-term facilitation induced by behavioral sensitization in Aplysia involves CREB-like proteins, as well as the immediate-early gene ApC/EBP. Using the bZIP domain of ApC/EBP in a two-hybrid system, we have cloned ApCREB2, a transcription factor constitutively expressed in sensory neurons that resembles human CREB2 and mouse ATF4. ApCREB2 represses ApCREB1-mediated transcription in F9 cells. Injection of anti-ApCREB2 antibodies into Aplysia sensory neurons causes a single pulse of serotonin (5-HT), which induces only short-term facilitation lasting minutes, to evoke facilitation lasting more than 1 day. This facilitation has the properties of long-term facilitation: it requires transcription and translation, induces the growth of new synaptic connections, and occludes further facilitation by five pulses of 5-HT.

Activating Transcription Factor 2↗
PubMed1995

Evidence for synaptotagmin as an inhibitory clamp on synaptic vesicle release in Aplysia neurons.

While previous studies have demonstrated that synaptotagmin plays an essential role in evoked neurotransmitter release, it has been difficult to determine whether it acts to facilitate or inhibit release. To address this question, we used acute genetic manipulations to alter the expression of synaptotagmin in Aplysia neurons. Transient overexpression of synaptotagmin in acutely dissected cholinergic neurons and in cultured glutaminergic neurons decreased the amplitude of the excitatory postsynaptic potential (EPSP) by 32% and 26%, respectively. In contrast, treatment of cultured presynaptic neurons with synaptotagmin antisense oligonucleotides increased the amplitude of the EPSP by 50-75%. These results are consistent with a role of synaptotagmin as an inhibitor of release.

Amino Acid Sequence↗
PubMed1995

A VAMP-binding protein from Aplysia required for neurotransmitter release.

Before the fusion of synaptic vesicles with the plasma membrane, a protein complex is thought to form between VAMP--an integral membrane protein of the vesicle--and two proteins associated with the plasma membrane, SNAP-25 and syntaxin. The yeast two-hybrid interaction cloning system has now been used to identify additional proteins from Aplysia that interact directly with VAMP. A 33-kilodalton membrane protein, termed VAP-33 (VAMP-associated protein of 33 kilodaltons), was identified whose corresponding messenger RNA was detected only in the central nervous system and the gill of Aplysia. Presynaptic injection of antibodies specific for VAP-33 inhibited synaptic transmission, which suggests that VAP-33 is required for the exocytosis of neurotransmitter.

Amino Acid Sequence↗
PubMed1995

Hippocampal long-term depression and depotentiation are defective in mice carrying a targeted disruption of the gene encoding the RI beta subunit of cAMP-dependent protein kinase.

The cAMP-dependent protein kinase (PKA) has been shown to play an important role in long-term potentiation (LTP) in the hippocampus, but little is known about the function of PKA in long-term depression (LTD). We have combined pharmacologic and genetic approaches to demonstrate that PKA activity is required for both homosynaptic LTD and depotentiation and that a specific neuronal isoform of type I regulatory subunit (RI beta) is essential. Mice carrying a null mutation in the gene encoding RI beta were established by use of gene targeting in embryonic stem cells. Hippocampal slices from mutant mice show a severe deficit in LTD and depotentiation at the Schaffer collateral-CA1 synapse. This defect is also evident at the lateral perforant path-dentate granule cell synapse in RI beta mutant mice. Despite a compensatory increase in the related RI alpha protein and a lack of detectable changes in total PKA activity, the hippocampal function in these mice is not rescued, suggesting a unique role for RI beta. Since the late phase of CA1 LTP also requires PKA but is normal in RI beta mutant mice, our data further suggest that different forms of synaptic plasticity are likely to employ different combinations of regulatory and catalytic subunits.

Animals↗
PubMed1995

Focal adhesion kinase in the brain: novel subcellular localization and specific regulation by Fyn tyrosine kinase in mutant mice.

Signaling by tyrosine kinases is required for the induction of synaptic plasticity in the central nervous system. Comparison of fyn, src, yes, and abl nonreceptor tyrosine kinase mutant mice shows a specific requirement for Fyn in the induction of long-term potentiation at CA1 synapses in the hippocampus. To identify components of a Fyn-dependent pathway that may be involved with hippocampus function we examined tyrosine-phosphorylated proteins in kinase mutant mice. We found that nine proteins were hypophosphorylated specifically in fyn mutants. One of the hypophosphorylated proteins was focal adhesion tyrosine kinase (FAK). FAK also showed reduced activity in immunocomplex kinase assays only in fyn mutants. FAK is expressed at very high levels in the brain but in contrast to non-neural cells, FAK was not restricted to focal adhesion contacts. FAK was found in axons, dendrites, and the intermediate filament cytoskeleton of astrocytes. Brain extracts from the mutants also show specific patterns of compensatory changes in the activity of the remaining Src family kinases. Tyrosine phosphorylation is a critical regulator of FAK, and impairments in FAK signal transduction in fyn mutants may contribute to the mutant neural phenotype.

Animals↗
PubMed1995

Activity-dependent long-term enhancement of transmitter release by presynaptic 3',5'-cyclic GMP in cultured hippocampal neurons.

Long-term potentiation (LTP) in hippocampus is a type of synaptic plasticity that is thought to be involved in learning and memory. Several lines of evidence suggest that LTP involves 3',5'-cyclic GMP (cGMP), perhaps as an activity-dependent presynaptic effector of one or more retrograde messengers (refs 2-12, but see ref. 13). However, previous results are also consistent with postsynaptic effects of cGMP. This is difficult to test in hippocampal slices, but more rigorous tests are possible in dissociated cell culture. We have therefore developed a reliable method for producing N-methyl-D-aspartate (NMDA) receptor-dependent LTP at synapses between individual hippocampal pyramidal neurons in culture. We report that inhibitors of guanylyl cyclase or of cGMP-dependent protein kinase block potentiation by either tetanic stimulation or low-frequency stimulation paired with postsynaptic depolarization. Conversely, application of 8-Br-cGMP to the bath or injection of cGMP into the presynaptic neuron produces activity-dependent long-lasting potentiation. The potentiation by cGMP involves an increase in transmitter release that is in part independent of changes in the presynaptic action potential. These results support a presynaptic role for cGMP in LTP.

Action Potentials↗