BIPN140 Lecture 12: Synaptic Plasticity (II) 1. Early v.s. Late LTP 2. Long-Term Depression 3. Molecular Mechanisms of Long-Term Depression: NMDA-R dependent 4. Molecular Mechanisms of Long-Term Depression: NMDA-R independent (cerebell LTD) Su (FA16) Mechanism II: Ca 2+ influx is Required for LTP (Fig. 8.13) Early phase of LTP (first hour or two) The requirement for NMDA receptors for LTP suggest that Ca 2+ influx is critical for LTP induction. Evidence (necessity): Ca 2+ is needed in the postsynaptic neuron for LTP => infusion of Ca 2+ chelators (e.g. BAPTA) via patch pipet blocks LTP. Evidence (sufficiency): Photolysis of caged Ca 2+ together with subthreshold presynaptic tetanus can induced LTP. Postsynaptic molecular mechanisms: (1) Ca 2+ entry activates postsynaptic kinases, such as CaMKII and PKC. (2) Recruit intracellular AMPA receptors to surface, thus increasing EPSP amplitude. CaMKII is the most abundant postsynaptic protein at Schaffer collateral synapses. Pharmacological inhibition or genetic deletion of CaMKII prevents LTP.
Dynamic AMPA-R Trafficking During Synaptic Plasticity AMPARs are highly mobile and traffic rapidly between membrane compartments within the plasma membrane. AMPARs rapidly move laterally in the extrasynaptic plasma membrane (very mobile) and can enter and exit synapses where they interact with scaffold proteins within the postsynaptic density (PSD) to immobilize them and concentrate them at the synaptic membrane. AMPA-R auxiliary subunit and the synaptic scaffolding protein, PSD-95, play an important role in immobilizing AMPA-Rs at synapses. AMPARs can be endocytosed for degradation in the endosomes or for recycling back to the plasma membrane. Huganir & Nicoll, Neuron 80, 704-717, 2013 This trafficking is highly regulated during LTP and LTD, resulting in increases or decreases of AMPARs at the synapse (synaptic pools <-> non-synaptic pools). Scaffolding and Trafficking Proteins for AMPA-Rs Scaffolding proteins (e.g. PSD-95): bind to the C-termini of AMPA-Rs & NMDA-Rs; subcellular targeting of channel interacting partners. AMPA-R interacting partners: regulate synaptic targeting and membrane trafficking. Binding of AMPA-R subunits with the interacting partners is regulated by post-translational modification (phosphorylation & palmitoylation). Huganir & Nicoll, Neuron 80, 704-717, 2013
AMPA-R Phosphorylation and its Trafficking CaMKII on GluA1 => channel conductance Major forms of AMPA-Rs in the hippocampus: GluA1/2 and GluA2/3 heteromers as well as GluA1 homomers (permeable to Ca 2+, likely extra-synaptic). All AMPA-R subunits are phosphorylated on Ser & Thr by several kinases, including CaMKII, PKA, PKC, PKG, JNK etc. on over 20 different phosphorylation sites. Knock-in mice expressing GluA1 without CaMKII & PKA phosphorylation sites => deficit in LTP and LTD. Song & Huganir, TRENDS in Neuroscience 25, 578-588, 2002 Derkach et al, Nature Reviews Neuroscience 8, 101-113, 2007
Role of Protein Synthesis in Maintaining LTP (Fig. 8.14) Early LTP: induced by a single tetanus (100 Hz, 1 sec); last for 1-2 hours (weak tetanus only triggers early LTP) Late LTP: induced by 4 tetani, 10 minute apart; can last for >8 hours Early LTP is not blocked by anisomycin, an inhibitor of protein synthesis, but late LTP is affected. Kandel et al., Principles of Neural Science, 5 th Edition Mechanisms Responsible for Long-lasting Changes in Synaptic Transmission during LTP (Fig. 8.15) LTP-induced structural changes CA1 pyramidal neurons Early LTP: protein kinases => post-translational changes in membrane channels Late LTP: repetitive strong tetani => Ca 2+ influx => adenylyl cyclase => PKA => MAPK cascade => CREB phosphorylation (activation) => transcription/translation of new genes (transcription factors, protein kinases, AMPA-Rs) => (1) maintaining LTP in the stimulated synapses and/or (2) growth of new synapses.
Synaptic Tagging & Late LTP Only strong tetanus can trigger late LTP (additional intracellular signaling that triggers transcription/translation) How does the neuron know to which synapse the newly synthesized protein (diffusible plasticity-related proteins, PRPs) should be delivered? Local tag is produced at the specific synapse that was activated during tetanus (it turns out that tagging does not require very strong tetanus). PRPs are then captured by tagged synapses; responsible for late LTP. PRPs disappears in about 2-3 hours (duration of early LTP). Supported by the observation of associative late LTP. Long-term Synaptic Depression in the Hippocampus (Fig. 8.16) Low frequency stimulus (e.g. 1 Hz for 10-15 mins) 1. Trigger: prolonged low frequency stimulus, e.g. 1 Hz (vs 100 Hz for LTP). 2. Mechanism: calcium-dependent activation of phosphatases; internalization of AMPA receptors. 3. NMDA receptor is also required (APV blocks LTD as well): certain NMDA subunit composition (channel stoichiometry) may favor LTD over LTP. 4. Different intracellular Ca 2+ dynamics: slow rise => preferentially activate phosphatase (high Ca 2+ affinity). In contrast, a large intracellular Ca 2+ surge favors LTP. 5. Physiological role: homeostasis, prevent saturation of LTP (otherwise can t learn any more), forgetting (behavioral flexibility, i.e. learning new things). 6. LTD is also input/synapse specific.
Synaptic Efficacy Can Be Regulated Bidirectionally TBS: Theta burst stimulation, high frequency tetanus to induce LTP LFS: Low frequency stimulation for 15 minutes to induce LTD. LFS can eliminate LTP (or induce LTD) as short as 30 minutes after TBS (LTP induction). Synaptic efficacy can be dynamically tuned up or down (AMPA-R trafficking in and out of synapses). Dudek & Bear, J Neuroscience 13, 2910-2918, 1993 Spike-timing Dependent Synaptic Plasticity (STDP) (Fig. 8.18) Action potential superimposed on EPSP LTD LTP The precise temporal relationship between activity in the pre- and post-synaptic neurons is also an important determinant of LTP/LTD. STDP enhancement of LTP: presynaptic AP before postsynaptic AP => bigger LTP Likely mechanism: augmented opening of NMDA receptors & Ca 2+ influx. STDP enhancement of LTD: presynaptic AP after postsynaptic AP => more LTD (no LTP) Likely mechanism: reduced activation of NMDA receptors/reduced Ca 2+ influx; retrograde diffusible signals (e.g. endocannabinoid, to reduce SV release from presynaptic terminals, remember depolarization-induced suppression of inhibition in lecture 8). Action interval: <40 ms Implications: (1) STDP can provide a means of encoding information about causality. (2) STDP could also serve as a mechanism for competition between synaptic inputs: stronger inputs would be more likely to be reinforced by LTP, and weaker inputs further weakened by LTD.
Cerebellar LTD (Fig. 8.17) (smaller EPSPs) Cerebellar Purkinje cells receive two main excitatory glutamatergic inputs from (1) parallel fiber (granule cells, small excitatory input, spatially restricted) and (2) climbing fiber (neurons of the inferior olive, very strong excitatory input, large EPSPs). Coincidence detection in postsynaptic Purkinje cells of concurrent inputs from parallel fibers and climbing fibers => LTD in synaptic input from parallel fibers (weak => weaker). (strong excitatory input) Form inhibitory synapses onto cerebellar output neurons (DCN) It has been postulated that cerebellar LTD underlies certain forms of motor learning, particularly associative eye blink conditioning. That is, cerebellar LTD is a form of LTD that mediates learning (not forgetting). Unconditioned stimulus (US): a puff of air to the eye, causing eye blink (climbing fiber stimulus). Conditioned stimulus (CS): auditory stimulus immediately before air puff (from mossy fiber => granule cells => parallel fibers) CS before US several times => animals learn to blink in response to auditory stimulus. Cerebellar LTD (Fig. 8.17) Coincidence detection in postsynaptic Purkinje cells of concurrent inputs from parallel fibers and climbing fibers. (1) Parallel fiber input: AMPA receptors and metabotropic GluRs (increase IP3 and DAG) (2) Climbing fiber input: VGCCs allow Ca2+ influx (strong excitatory input). (3) Consequence: PKC activates kinase; phosphorylated AMPA receptors (GluA2) internalize. The availability of AMPA receptors determines the strength of the synapse. The temporal interplay between the electrical and chemical signals allows the Purkinje cells to integrate different inputs to modulate the strength of synaptic inputs from PFs (the basis of learning and memory) Different from hippocampal LTD: does not require NMDA receptors (NMDA-independent LTD); phosphorylation (kinase, PKC) instead of de-phosphorylation (phosphatase) drives AMPA receptor internalization.
Cerebellar LTD: Mechanism Climbing Fiber Voltage-Gated Ca 2+ Channel Ca 2+ AMPA Receptor Phosphorylation Internalization LTD Parallel Fiber IP 3 Ca 2+ mglur1=> PLC DAG PIP 2 PKC
Background: Long-term modification of synaptic strength (LTP & LTD) has long been postulated to encode memory. However, the causal link between LTP/LTD and memory has been difficult to demonstrate. Experiments: Using fear conditioning (a type of associative memory) as a paradigm to study the impact of optogenetically induced LTP & LTD on fear memory. Expressing channelrhodopsin2 (ChR2, a light-gated non-selective ion channel, optogenetic approach) in the neurons in the auditory cortex that project to amygdala (fear center in the brain). (1) Using light to induce LTD after fear conditioning. (2) Using light to induce LTP subsequently to determine its impact on fear memory. Results: Optogenetic delivery of LTD conditioning to the auditory input inactivates memory of the foot shock. Conversely, subsequent optogenetic delivery of LTP conditioning to the auditory input reactivates memory of the shock. Thus, the authors engineered inactivation and reactivation of a memory using LTD and LTP, which supports a causal link between these forms of synaptic plasticity and memory. Fig. 1. Fear conditioning with tone or optogenetics. US: unconditioned stimulus, foot shock 10 Hz light flashes Lateral amygdala 1. Animals are trained to press a lever in response to CS (conditioned stimulus, tone or optogenetically driven input, ODI). 2. When animals experience fear, they freeze and stop pressing the lever. +40 mv 0 mv -60 mv 1. Neurons expressing ChR2 respond to blue flashes faithfully with action potential (up to 100 Hz). 2. Provides a means to induce LTD or LTD in vivo in behaving animals.
Fig. 2. LTD inactivates and LTP reactivates memory (1 Hz x 900 pulses) (100 Hz x 100 pulses x 5) Fig. 4. In vivo electrophysiological responses to 10 Hz (baseline), LTD and LTP protocols. Head fixed, anaesthetized animals, extracellular field recording (field EPSP slope) (optical conditioned stimulus)
Results: Optogenetic delivery of LTD conditioning to the auditory input inactivates memory of the foot shock. Conversely, subsequent optogenetic delivery of LTP conditioning to the auditory input reactivates memory of the shock. Thus, the authors engineered inactivation and reactivation of a memory using LTD and LTP, which supports a causal link between these forms of synaptic plasticity and memory. Cerebellar Circuitry and Eye Blink Conditioning (+) (+) (+) (-)(-) (+) (+) Premotor area
Kandel et al., Principles of Neural Science, 5 th Edition