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LONG-TERM POTENTIATION (LTP) Introduction LTP as a candidate mechanism for the activity-dependent change in the strength of synaptic connections LTP is

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  • LONG-TERM POTENTIATION (LTP)Introduction LTP as a candidate mechanism for the activity-dependent change in the strength of synaptic connections LTP is a persistent increase in synaptic strength (as measured by the amplitude of the EPSP) that can be rapidly induced by brief neural activity.

  • Anatomical background for Hippocampus

    two interlocking C-shaped regions (the hippocampus and the dentate gurus), main inputs: entorhinal cortex three major afferent pathways (subiculum -> CA1) Perforant pathway (subiculum -> granule cells in dentate gyrus) Mossy fiber pathway (axons of the granule cells -> pyramidal cells in the CA3) Schaffer collaterals (pyramidal cells in the CA3 -> pyramidal cells in the CA1)

  • Perforant pathway (subiculum -> granule cells in dentate gyrus) Mossy fiber pathway (axons of the granule cells -> pyramidal cells in the CA3) Schaffer collaterals (pyramidal cells in the CA3 -> pyramidal cells in the CA1) entorhinal cortex subiculumdentate gyrusCA3CA1Perforant pathway Mossy fiber pathwaySchaffer collaterals

  • The initial finding by Timothy Bliss and Terje Lomo (1973) Anaesthetized rabbit Brief, high-frequency stimulation of the perforant pathway input to the dentate gyrus produced a long lasting enhancement of the extracellular recorded field potential.

  • Recording techniques In vivo (in awake and freely moving animals, or in anesthetized animals) in vitro (slice preparations) Extracellar recordings intracellular recordings

    Experimental designStimulation of a bundle of presynaptic axons recording of monosynaptic EPSP

    Typical results for induction of LTP

  • beforeafter

  • The "classical properties" of LTP Cooperativity The probability of inducing LTP, or the magnitude of the resulting change, increases with the number of stimulated afferents. Associativity associativity was shown in preparations in which two distinct axonal inputs converged onto the same postsynaptic target Concurrent stimulation of weak and strong synapses to a given neuron strengthens the weak ones. Input specificity LTP is restricted to only the inputs that received the tenanic (high-frequency) stimulation

  • Underlying molecular mechanisms Introduction LTP requires some sort of additive effect High-frequency stimulation Activation of synapses and depolarization of the postsynaptic neuron must occur at the same time LTP (in area CA1) depends on certain changes at glutamate synapses, Types of glutamate receptors NMDA receptors Non-NMDA receptors, AMPA At non-NMDA receptors, glutamate is excitatory Open channels for sodium ions

  • At NMDA receptors

    BEFOREControls a calcium ion channel glutamate is neither excitatory nor inhibitory Ion channel is blocked by magnesium ions

    DURING INDUCTION Activation of NMDA receptors requires both glutamate and depolarization, which lead to the removal of magnesium ions The NMDA receptors now respond actively to glutamate and admit large amount of Ca2+ through their channels After induction of LTP, transmission at non-NMDA receptors is facilitated (entry of Na+)

  • LTP is induced via a cascade of neurochemical stepsThe entry of Ca2+ ions into neurons activates some protein kinases (which are enzymes that catalyze phosphorylation, the addition of phosphate groups to protein molecules). One of the kinase, Calcium-calmodulin kinase (CaM kinase) remains activated once it is put into that state by Ca2+, even if the level of Ca2+ subsequently falls The activated protein kinases also trigger the synthesis of proteins activate cAMP responsive element-binding protein (CREB) CREB -> production of the transcription (mRNA) of immediate early genes (IEGs) -> regulate the expression of particular late effector genes (LEGs) -> synthesis of proteins Induction of LTP requires a retrograde signal, from the postsynaptic neuron to the presynaptic neuron