Long-Term Potentiation (LTP) PDF

Historical background leading up to the discovery of long-term potentiation (LTP) DONALD HEBB Contributions 1. Cell assembly theory: neural representation of cells modified ensembles of neurons could provide a substrate for memories Sensory inputs into a distributed set of w...

Historical background leading up to the discovery of long-term potentiation (LTP) DONALD HEBB Contributions 1. Cell assembly theory: neural representation of cells modified ensembles of neurons could provide a substrate for memories Sensory inputs into a distributed set of weakly connected collection of cell assemblies change the strength of connections among the neurons in the assemblies 2. Hebbian plasticity: cells that fire together wire together Weak connections will be strengthened as stimulus activates them Synaptic Plasticity Theory: changes due to learning and memory are represented by synaptic changes (LTP & LTD) The MTL Patient H.H. with bilateral hippocampal removal = impaired declarative memory NEUROANATOMY OF MTL PrC: objects & PHC: scenes > EC: spatial layouts (Grids) > Perforant path > Dentate gyrus (Hipp) > mossy fibers > CA3 pyramidal cells (Hipp) > Schaeffer collateral axons > CA1 pyramidal cells (Hipp) > Subicular complex (Hipp) Fundamentals of LTP Long-term potentiation was the first evidence of persistent changes in activity in the brain following a manipulation How is synaptic strength modi ed? Tetanic stimulation leads to long-term potentiation of synaptic strength (Bliss & Lomo) Methods: stimulated the perforant path (SE) and recorded (RE) in the dentate gyrus Results: A weak stimulus (WS) leads to low synaptic activity A strong stimulus (SS) leads to high synaptic activity A WS applied after the SS leads to higher synaptic activity (compared to baseline): they referred to this as long-term potentiation (LTP) fi Three key questions: 1. What happens during the tetanus (high frequency stimulation) of stimulation that facilitates LTP: induction of LTP 2. How are synapses altered following LTP: expression of LTP 3. Are these processes important in learning? IS THE CHANGE IN SYNAPTIC STRENGTH SPECIFIC TO THE SYNAPSES OF THE STIMULATED PATHWAYS? Specificity: A strong stimulus results in LTP at synapse A, but not in inactive pathways (other synapses) If there is no activity at synapse (B) when a strong stimulus activates a synapse (A), there is no strengthening of synapses Associativity: A strong stimulus results in LTP at synapse A as well as pathways with weak stimulation (other synapses) If there is even a little activity to synapse (B) when a strong stimulus activates a synapse (A), then there will be strengthening of synapses Methodology: Studying LTP in Tissue Slices Taken from the Hippocampus This procedure is called an in vitro preparation: chamber filled with CSF needed to keep the slice viable a small chamber that holds the slice stimulating electrode (SE) used to induce LTP recording electrode (RE) used to measure: the field excitatory post-synaptic potential (EPSP) (extracellular recording) single EPSP (intracellular recording) PRINCIPLES OF ELECTROPHYSIOLOGY Membrane potential is measured by determining the difference in voltage across two electrodes 1 electrode intracellularly 1 electrode extracellularly Zero potential: no difference in voltage 2 RE extracellularly Resting membrane potential: ~70mV (extracellular is more positive than intracellular) PRINCIPLES OF SLICE ELECTROPHYSIOLOGY The resting membrane potential is negative Depolarization occurs when the ionic composition of the intracellular fluid becomes less negative (positive ions flow in) - EPSP Hyperpolarization occurs when the ionic composition of the intracellular fluid becomes more negative (negative ions flow in/positive ions flow out) - IPSP ELECTROPHYSIOLOGY RECORDINGS OF POSTSYNAPTIC POTENTIALS Field EPSP (fEPSP): a measure of the strength of a population of synapses measure of the sum of extracellular recordings of nearby neurons slope is used as a measure of the fEPSP strength Measuring long-term potentiation Standard LTP protocol 1. Establish baseline 2. Induction stimulus (strong stimulation) 3. Check whether synaptic activity increased Dependent variable is a ratio: fEPSP (% of baseline) =T2/T1 times 100 What happens during the stimulation, or the induction phase? (Malenka) 1. Depolarization of the post-synaptic cell is required for LTP Activation of both pre- and post- neurons: post- depolarization via Glu from pre- 2. Tetanus is not required during depolarization of the post-synaptic cell 3. Hyperpolarization of the post-synaptic cell during the stimulation procedure fails to induce LTP What synaptic changes take place that produce LTP? A debate centered on 2 general possibilities: LTP is the result of: 1. presynaptic changes that increase the release of Glu 2. postsynaptic changes that increase the postsynaptic neuron’s sensitivity to Glu Although presynaptic changes do exist, it is safe to assume that important postsynaptic changes are essential to LTP (Nicoll, 2017; Vincent-Lamarre et al., 2018) The NMDA receptor in LTP Ionotropic receptors Ionotropic receptors are located in the plasma membrane When a NT binds to the receptor, the channel or pore opens and allows ions such as Na+ and Ca2+ to enter the cell Pharmacological agents can be used to enhance (agonist) or inhibit (antagonist) receptor function Competitive antagonist: binds reversibly to the active site of a R Blocks an agonist form binding to its receptor while maintaining its inactive state Noncompetitive antagonist: can bind to either the active or allosteric site Types of Gluergic Receptors AMPAR Glu binds to AMPAR NMDAR NMDA receptors are involved in induction but not expression LGIC but also voltage-gated Mg2+ channel Opening of NMDAR requires 2 events: Glu must bind to the receptor the cell must depolarize > the Mg2+ plug is removed (pop out) and Ca2+ can enter the cell (greater depolarization) LTP INDUCTION Activation of NMDA receptors requires pre- and post-synaptic activity (Malenka) Pre- : No Glu released > Post -: depolarization removes Mg2+ block = NO LTP Pre-: Glu released > Post-: hyperpolarization maintains Mg2+ block = NO LTP Pre-: Glu released > Post-: depolarization removes Mg2+ block = LTP Does LTP dependent on NMDA receptors mediate Hebbian learning? YES HOW DOES CA2+ INFLUX AT THE SYNAPSE LEAD TO LONG-TERM FACILITATION OF SYNAPTIC STRENGTH? If you remove Ca2+ in the synaptic cleft, you can block LTP Induction of LTP is post-synaptic LTP EXPRESSION EVIDENCE FOR POST-SYNAPTIC EXPRESSION: AMPA RECEPTORS Two pools of AMPAR: A pre-existing surface pool If blocked = block LTP early on/slow expression of LTP An intracellular pool If blocked, the earlier expression of LTP is the same but will rapidly decrease 2 Independent Processes Deliver AMPAR to the PSD When LTP is induced, AMPAR get greater pre- surface & recruit intracellular AMPAR to the synapse = greater synapse strength 1. lateral diffusion of receptors in the surface pool into the PSD (post-synaptic density) 2. receptors from the intracellular pool are delivered by motor proteins Can become locked into the synapse If you inhibit both process = prevent LTP AMPAR recruitment during LTP AMPAR to the PSD through other proteins: Ser831 If phosphorylated = allows more Na+ into the cell = more excitable = LTP Ser845 if phosphorylated = tag R for other proteins to remove AMPAR from the synapse = less excitable = no LTP/LTD Switch AMPAR from GluR1 to GluR2 (no Mg2+ block) = always allow Ca2+ into the cell (no need for NMDA) How does Ca2+ lead to synaptic changes? Ca2+ > Calmodulin > CaMK2 (memory protein) CaMK2 is required for expression of LTP & why LTP can last for a long time Kinase: adds a phosphate group (-PO4) to a protein, which induces a conformational change in the protein and activates it Phosphatase: removes a phosphate group (-PO4) from a protein, which induces a conformational change in the protein and resets it to its resting (inactivated) state ACTIVATION OF CAMK2 1. CaMK2 is bound to actin in dendritic spines when inactive 2. CaM2 binds to Ca2+ when active 3. Recruits more actin into the synapse (enlarge the synapse) and to recruit more AMPAR on the synapse Disruption of actin regulation, the synapse will get smaller and AMPAR will be removed LARGER SPINES ARE MORE STABLE Small spines are not persistent Larger spine endure for longer periods of times Size correlates with number of AMPAR Stability of LTP LTP is susceptible to disruption in the first 10 min by low frequency stimulation (LFS) LFS prevents AMPAR recruitment After 10 min, the spines are large enough and stable enough to not be disrupted LOCAL SYNAPTIC CHANGES UNDERLYING LTP 1. Ca2+ enters the cell and activates CaMK2 2. AMPAR are recruited to the spine via lateral diffusion and then via intracellular pools 3. CaMK2 promotes spine growth 4. Larger spines recruit more AMPAR via actin 5. Recruitment of AMPAR is very susceptible to disruption in the first 10 min Associativity & LTP Associativity: A strong stimulus results in LTP at synapse A as well as pathways with weak stimulation (other synapses) If there is even a little activity to synapse (B) when a strong stimulus activates a synapse (A), then there will be strengthening of synapses (B) If changes are local at the synapse, how do other weak synapses get strengthened? Protein synthesis hypothesis Protein synthesis inhibitors injected into the hippocampus prevent memory consolidation Pre-treatment with anisomycin (protein synthesis inhibitor) prevents the HFS from producing long-lasting LTP Strength of stimulation and LTP Different strength of stimulation lead to different durations of LTP: LTP1: weak (1 TBS) induces local Ca2+ influx into the spine and some intracellular stores but not enough for protein synthesis LTP2: stronger (4 TBS) stimulation leads to further release of intracellular Ca2+ and lead to protein synthesis LTP3: strongest (8 TBS) repeatedly opens VG-Ca2+ on cell body = transcription of genes for expression of LTP at the spines Binds to CREB in soma CREB binds to promoter that are important for synaptic plasticity MRNAS TRANSCRIBED NEED TO KNOW WHICH SYNAPSES THEY GO TO MODIFY Synaptic tag & capture hypothesis: tag happens at the synapses of a strong stimulation occurs that tells mRNA where to go WS = only tags synapses SS = tags + generates PPs needed for LTP Evidence (Frey) WS = short lasting LTP SS = long lasting LTP SS + WS = long lasting LTP in weak synapses Tag primes the weak synapses Long-Term Depression (LTD) = the opposite of LTP Need a process to regulate excitation Neurons are only made to work at an optimized level of APs LFS induces a low fEPSP slope Activates Ser845 to remove AMPAR from the membrane Homeostatic plasticity: neurons will open VG-Cl- channels to that negative ions enter the cell and reduce excitability