BIPN140 Lecture 11: Synaptic Plasticity (I) 1. Short-Term Synaptic Plasticity 2. Long-Term Synaptic Plasticity 3. Molecular Mechanisms of Long-Term Potentiation Su (FA16) Synaptic Plasticity Synaptic plasticity: the connection between neurons are subject to modification (development, activity, physiological state, experience, disease, drugs, etc.). Changes in synaptic efficacy are widely believed as the basis for: (1) certain forms of learning (2) storage of memories (3) refinement of connections during development How to modify the strength/nature of connection between neurons? (1) changes in the number of synapses (2) changes in the probability of NT release (presynaptic mechanism) (3) changes in the number and/or property of NT receptors (postsynaptic mechanism) Synaptic plasticity varies in time scale from milliseconds to years. Short-term Synaptic Plasticity Most short-term synaptic plasticity (milliseconds to minutes) affect the amount the neurotransmitter released from presynaptic terminals. Key regulators: frequency of presynaptic APs, presynaptic [Ca2+] /sensitivity to Ca2+, availability of readily releasable SVs. Four major types: (1) Synaptic facilitation: ms, Ca2+ build up due to quick succession of APs (2) Synaptic depression: ms, transient depletion of readily releasable SVs. (3) Synaptic augmentation: seconds, more effective use of Ca2+. (4) Post-tetanic potentiation (PTP): seconds to minutes, better mobilization of SV pools (more phosphorylation of synapsin to mobilize SVs) Synaptic Facilitation (Fig. 8.1) AP interval A rapid increase in synaptic strength that occurs when two or more APs invade the presynaptic terminal within a few milliseconds. Facilitation by the 1st AP (conditioning pulse) lasts for tens of milliseconds. Prolonged elevation of presynaptic Ca2+ level (Ca2+ influx is faster than Ca2+ removal), resulting in Ca2+ buildup. 2+ n At NMJ, EPP amplitude = K([Ca ]o) , n=3.8 (Four Ca2+ ions cooperate to cause release of synaptic vesicles) Possible molecular mechanism: cooperativity of synaptotagmin-Ca2+ binding, partial occupancy of the Ca2+ binding site enhances further Ca2+ /phospholipid binding. PSPs rise Paired-pulse facilitation: 2+ depends on AP frequency, sharply as Ca increases. Ca2+ concentration Favorable experimental condition for synaptic facilitation: lowering external Ca2+. Why? Synaptic Depression (Fig. 8.1) 2+ Decline of neurotransmitter release during sustained [Ca ]out = 0.28 mM synaptic activity. 2+ [Ca ]out = 2.5 mM The occurrence/degree of facilitation or depression depends on extracellular Ca2+ concentration. High frequency APs Depression depends on the amount of neurotransmitter that has been released. Possible mechanism: depression is caused by progressive depletion of a pool of synaptic vesicles that are available for release. Experimental support: more depression is observed after the size of the reserve pool is reduced in synapsin knockout animals. Paired-pulse depression: also depends on AP frequency Augmentation & Potentiation (Fig. 8.1) Forms of short-term plasticity that occur over a slightly longer time scale (seconds to minutes). Elicited by repetitive synaptic activity that lead to prolonged elevation of presynaptic Ca2+. Function: increase the amount of neurotransmitter released from presynaptic terminals. Both enhance the efficacy of Ca2+ to trigger SV release. Augmentation: rises and falls over a few seconds (possibly regulated by munc-13, a priming factor, PLC dependent, increase the size of readily releasable SV pool and release probability ) Potentiation: acts over tens of seconds to minutes (via kinases that phosphorylate substrates such as synapsin to increase the size of readily releasable SV pool and release probability). Short-term Plasticity at the Neuromuscular Synapse (Fig. 8.2) During repetitive synaptic activity, different forms of short-term plasticity can interact to cause complex & dynamic changes of synaptic activity. Readily releasable SVs: if many are still available, facilitation (immediate) and augmentation (slightly later). If depleted, synaptic depression. Later on, PTP increases the size of readily releasable pool to further potentiate the synapse. Synaptic transmission can change dynamically as a consequence of the recent history of synaptic activity (context or experience). Long-term Synaptic Plasticity Long-term synaptic plasticity alter synaptic transmission over time scales of 30 minutes or longer. Arises from molecular mechanisms that vary over time (short to long): (1) post-translational modifications (e.g. phosphorylation) of existing proteins (changes in AMPA receptor trafficking) (2) changes in translations (local translation) (3) changes in gene transcription + translation (producing enduring changes in synaptic transmission). Two major forms: (1) long-term potentiation (LTP): long-lasting increase in synaptic strength (2) long-term depression (LTD): long-lasting decrease in synaptic strength Changes in synaptic efficacy that are likely the basis for: (1) certain forms of learning (2) storage of memories (3) refinement of synaptic connections during development Hippocampus (Fig. 8.6) A cortical structure in the medial portion of the temporal lobe (medial temporal lobe in primates). Critical for the consolidation of information from short-term memory to long-term memory and spatial navigation & spatial memory. Different types of neurons and collaterals (axon bundles) are organized in an orderly and relatively planar manner that is conducive to slice preparation electrophysiology. Most of the relevant circuitry is left intact in a hippocampus slice. The Tri-synaptic Circuit of the Hippocampus (Fig. 8.6) Tri-synaptic circuit: 3 axon bundles (perforant path, mossy fibers & Schaffer collaterals) + 3 main neuronal types (granule cells, CA3 & CA1 pyramidal neurons) Input from entorhinal cortex (via perforant path) => granule cell (via mossy fibers) => CA3 pyramidal cell (via Schaffer collaterals) => CA1 pyramidal cell. Excitatory glutamatergic connections. Clusters of monosynaptic connections. CA: cornu ammonis (ram’s horn) Long-term Potentiation (LTP) Extracellular recording Granule cells High frequency stimulation Perforant path Inducing LTP: a tetanus (high-frequency A few seconds of high-frequency electrical electrical stimulus, e.g. 100 Hz). stimulation can enhance synaptic transmission in the rabbit hippocampus for Recording: extracellular recording to days. measure amplitude or the slope of field potential (collective EPSPs) or intracellular patch-clamp recording to measure the amplitude of EPSPs. LTP is Input-Specific and Persistent (Fig. 8.7) Tetanus Tetanus stimulus at Schaffer collaterals, patch- clamp recordings at CA1 pyramidal neurons. If the Schaffer collaterals are stimulated only 2-3 times per minute, the size of the evoked EPSP in the CA1 neurons remain constant (pre-tetanus stimulus, baseline EPSP). Tetanus at pathway 1 induced LFP when pathway 1 In vivo recording with embedded electrode is stimulated. As a control, activation of pathway 2 (no tetanus) does not yield EPSPs of enhanced amplitude (no LTP) on the same postsynaptic neuron. LTP is input-specific. LTP can last for a long time! LTP is Associative (Fig. 8.9) S1 From Bliss & Collingridge (1993), Nature 361, 31-39. S1 is the weak input (low intensity), while S2 is (PTP) the strong one (high intensity). Tetanus in S1 does not do much to EPSPs (no LTP), but S2 S1+S2 together result in LTP in both synapses. A form of hetero-synaptic LTP. Associative learning? Classical Pavlovian conditioning. Independent stimulation Coordinated stimulation of pathway 1 & 2 of pathway 1 & 2 Weak stimulation of pathway 2 alone does not trigger LTP. However, when the same stimulus to pathway 2 is activated together with strong activation of pathway 1, both sets of synapses are strengthened. LTP involves Coincidence Detection (Fig. 8.8) Associative LTP suggests that LTP occurs when both presynaptic and postsynaptic neurons are active. That is, the presynaptic neurons fires action potentials and the postsynaptic membrane is depolarized. (in the CA1 neurons) To test, low intensity tetanus at Schaffer collateral => no LTP in CA1 neurons. Paired the low-intensity stimulus with strong depolarization of the CA1 neurons, within 100 ms of neurotransmitter release from the Schaffer collaterals => result in LTP. LTP occurs only when presynaptic action potentials and postsynaptic depolarization are tightly linked in time => a coincidence detector. LTP: Potential for Information Storage Input-specific: when LTP is induced by activation of one synapse, it does not occur in other, inactive synapses that contact the same neuron. Selective storage of information likely takes place at synapses, not neurons. State-dependent: pairing of pre- and postsynaptic events, coincidence detector. Hebbian Principle: Donald Hebb, a Canadian psychologist, in 1949 stated “when an axon of cell A is near enough to excite a cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A ' s efficiency, as one of the cells firing B, is increased”. In other words, coordinated activity of a presynaptic terminal and postsynaptic neuron would strengthen the synaptic connection between them, as observed in LTP much later. A simplified version of Hebb’s rule: neurons that fire together wire together. LTP at synapses shows many properties for learning. Mechanism I: NMDA Receptors