Transcript Slide 1
Learning and memory in Aplysia Review: one method of learning mediated by presynaptic spike broadening Presynaptic facilitation by stimulation Presynaptic facilitation by serotonin application Presynaptic facilitation by cAMP injection Model: Serotonin mediates presynaptic spike broadening Associative learning: Differential conditioning produces more presynaptic facilitation Reduced experimental preparation Results: •CS+ greatly enhanced •CS- modestly enhanced •Compared to CS+ and US-only (sensitization). •Keep in mind that this fig is highlighting EPSP changes in MNs. SN spike broadening is present but not evident in the fig. Experimental procedures Correlating brain (at the level of synaptic facilitation) and behavior •Pairing greatly enhances synaptic facilitation relative to sensitization (US only) •Thus sensitization affects facilitation but associative processes appears amplified relative to sensitization. •Is it the same molecular process that is just ramped up? Co-activation mediated by SNs (CS) and serotonin from INs (via US) mediates associative effects •AKA activity-dependent presynaptic facilitation •Ca+ influx from CS stimulation binds to CM which binds to adenylyl cyclase and amps cAMP production •No Ca+, CM does not bind •Thus enhancement is dependent on coordinated activity produced from both the CS+ and the US •This is a mechanism for CS-US coincidence detection Is presynaptic facilitation really presynaptic? •Well yes but its all relative because the presynaptic element is associated with the postsynaptic side of an axo-axonic synapse with the FAC What about the Post synaptic side? •N-methyl-D-aspartate (NMDA) receptors •An excitatory but conditional glutamate receptor •Require glutamate and that the cell is depolarized to open •Provides another mechanism for CS-US coincidence detection X No CS O CS present Depolarized cell (due to CS) frees Mg++ from NMDA receptor allowing glutamate to act. Establishing NMDA as a post synaptic player Experimental paradigm •G1 Standard forward paring •G2 forward pairing plus blockade of NMDA with APV (receptor antagonist) •G3/G4 CS-only and CS-only plus APV •CS presented only in test phases for G3/G4. Results: MN EPSP enhancement is blocked by APV The fact that the CS+ effects are blocked by APV suggests that both pre and post synaptic changes are necessary for learning to be manifest. Combining pre and postsynaptic facilitation into one model Presynaptic effects (1): 1. 2. 3. 4. 5. US mediates 5HT Activates adenylyl cyclase convert ATP to cAMP Results in increased PKA activity Which decreases K+ efflux and spike broadening Presynaptic effects (2): 1. 2. 3. 4. 5. 6. CS mediates Ca influx Activates calmodulin Enhances adenylyl cyclase Ramps up conversion of ATP to cAMP Results in ramping of PKA Which leads to decreased K+ efflux and spike broadening Post synaptic effects 1. NMDA receptors open via combined glutamate reception from SN (CS) and depolarization from depolarization caused by the US But do these changes last? Evidence of Long term memory Behavior lasts Facilitation lasts Short vs long term facilitation may depend on where serotonin is received •When serotonin is applied to axon terminal facilitation is short term •When serotonin is applied to soma terminal facilitation is long term Results suggest 5HT-mediated gene regulation Nuclear transcription mediated by PKA phosphorylation CREB CRE = cAMP response element CREB= cAMP response element binding protein PKA must trans locate to nucleus: this implies that there must be a surplus of PKA coming from terminals or it is generated near the soma (or both). •Phosphorylated CREB initiates immediate early genes which can initiate a cascade of other late response genes to be transcribed •Adding foreign CREs (as a competitive antagonist of endogenous CRE) reduces the effect of nuclear PKA hence blocks 5HT effects. •Amplifying nuclear PKA enhances sensitivity to 5HT. Learning-mediated gene expression results in long term changes in axonal and dendritic connectivity/complexity