Exam 2 3/30/16 Range: 60-100 Average: 79.8 Exam 1 2/17/16 Range: 49-98 Average: 77
Per student performance comparison exam 1 vs. 2
The four basic stages of neurotransmission
The generation & release of a synaptic vesicle
Synaptic vesicles are recycled following exocytosis
From action potential to postsynaptic depolarization
Action potential via Na+ channels depolarizes the presynaptic cell to open PSM Ca2+ channels & promote vesicle fusion
AMPA-R: Na+/K+ channel; NMDA-R: Ca2+ channel NMDA-R required for postsynaptic depolarization
Remember: the NMJ synapse requires ACh - AChR
# docked vesicles (pre) + active Rs (post) Both presynaptic and postsynaptic factors influence release probability **Kandel 3rd ed Chap 65 p1009 this lecture See pg. 1013 Fig 65-3 for sensitiz’n, facilit’n # docked vesicles (pre) + active Rs (post) # release sites (pre) + active Rs (post) # active Rs & # spines (post) contacting AZ (pre)
Habituation vs. sensitization to repeated stimulus mild stimulus Habituation: repeated stimulus gives progressively reduced response (eg. EPSP)= neuronal output Sensitization: repeated stimulus gives heightened response after time; this mechanism is how an organism learns to respond to a noxious stimulus (= learned avoidance or adaptation) noxious stimulus Sensitization:
Aplysia as a model for learning and memory Aplysia californica: marine snail Eric Kandel utilized this organism because it has a simple gill withdrawal reflex defensive behavior that he could modulate and study in the laboratory Eric Kandel
Aplysia protects itself from potential harm by withdrawing its gill when the siphon is touched
40 sensory neurons (siphon skin) synapse w/ 6 gill MNs & excitatory and inhibitory INs
Electrophysiology in Aplysia using the abdominal ganglia
Habituation was observed in Aplysia by EPSP recordings after repeated siphon stimulation Decrease in the open probability of Ca++ channels----decreased NT release as a result
Possible mechanisms for short-term habituation
Habituation leads to decreased neurotransmitter release and reduced gill withdrawal
Long-term habituation after 4 days of training synaptic depression & fewer sensorimotor synapses See Kandel Fig 65-2 p.1011 Long term habitutation 10 X 4 days
A strong aversive stimulus leads to enhanced neurotransmission via facilitatory INs amplified signal to MNs = sensitization See Kandel Fig.65-3 p.1013
Sensitization/short-term memory involves 5-HT, cAMP, & PKA All 3 increase Glu release
Increased MN response (EPSPs) after injecting 5-HT, cAMP, or PKA
5-min incubation with 5-HT causes cAMP increase (pre) + EPSP (post) = cAMP facilitates sensitization
Ionotropic Rs (eg. AMPA): ion channel; GABAB-R, 5HTR: metabotropic R
Decreased K+ via PKA phosphorylation prolongs action potential 1) 5-HT binds R; AdCyc ON 2) cAMP turns PKA ON 3) PKA phos. K+ channel– closed 4) action potential keeps Ca2+ channels open normal action potential *see Kandel p.1012-1013 Fig. 65-3 1) 5-HT binds 5-HTR to activate AdCyc; 2) cAMP increases activates cAMP-dep protK (PKA); 3) PKA phos. K+ channel to close it; 4) action potential continues to open Ca2+ channels after sensitization
PKA also acts directly on neurotransmitter release machinery
Presynaptic facilitation targets K+ channels, NT vesicles, & Ca2+ channels L-type Ca2+ channel: long-lasting voltage-gated channels
Repetitive shocks for 4 days induces shock memory for 2-3 weeks = LTM In left panel, control is touched w/ brush every 30 min; “one shock” animals receive a shock after 3rd brush touch [remember for 1h] In right panel, “4 single shocks” received 4 shocks/day & remembers for 2-3d; animals receiving 4 per day x 4d remember for 2-3 weeks= LTM!
Catalytic subunits of PKA translocate from the cytoplasm to the nucleus
RNA polymerase needs direct contact w/ enhancer binding proteins to activate transcription DNA in promoter & regulatory regions is thought to loop around to bring activating proteins in contact w/ polymerase
CREB-2 represses transcription; CREB-1 displaces it to activate CREB: cAMP response element binding protein; becomes phosphorylated by PKA then binds CRE; CBP then binds CREB to co-activate txn (CREB-CBP can also repress) CREB: basic Leu zipper protein complexing Mg2+
Sensitizing stimulus in tail results in heightened responses at the synapse & in behavior stimulating the tail then the siphon increased response in gill with drawal requires interneuron release of 5HT increased cAMP increased PKA enhanced NT release Sensitization causes increased synaptic potentials & heightened behavioral responses to stimulus
Long-term LTP: MAPK, CREB transcription/short-term LTP: 5HT, cAMP increases, activating PKA, closing K+ chan & incr NT release see Kandel p.1015 Fig. 65-5: short-term response to tail stimulation: 5HT interneuron activated AdCyc increase cAMP PKA activity shut off K+ channel, more Ca2+ for vesicle/NT release long-term response to tail stimulation: CREB binds to CREs & upreg transcription for synaptic stabilization, growth
Learning to pair stimulus with reward: classical conditioning
US/CS: unconditional/conditional stimulus Classical conditioning in Aplysia: pairing tail shock with water jet on the siphon; output= gill withdrawal reflex US CS US/CS: unconditional/conditional stimulus Mechanism: activation of interneurons via CS increases Ca2+ to enhance response to stimulus (activation of Ca2+-dep AdCyc)
Training and neuronal circuits in learning in Aplysia 5HT neuron (tail) siphon Conditioned stimulus: learned response (water on siphon) US: shock The animal learns to withdraw siphon for an interval
Conditioning: APCa2+ influx calmodulin cAMP PKA increased NT release
Presynaptic depolarization by INs increases Glu release to amplify PSP response
Figure 21-53. Intracellular signaling pathways during sensitization and classical conditioning in the Aplysia gill-withdrawal reflex arc. Sensitization occurs when the facilitator neuron is triggered by the unconditioned stimulus (US) in the absence of the conditioned stimulus (CS) to the siphon sensory neuron (see Figure 21-52). Classical conditioning occurs when the CS is applied 1 – 2 seconds before the US, and involves coincidence detectors in both the presynaptic siphon sensory neuron and the motor neuron. In the sensory neuron, the detector is an adenylate cyclase that is activated by both Ca2+-calmodulin and by Gsα· GTP (see Figure 21-42). In the motor neuron, the detectors are NMDA glutamate receptors (see Figure 21-40). Partial depolarization of the motor neuron induced by an unconditioned stimulus (via an unknown transmitter) from interneurons activated by the tail sensory neuron enhances the response to glutamate released by the siphon sensory neuron.
Learning & memory w/ odor + shock in Drosophila melanogaster
Genetic mutants in Drosophila that identified memory pathways amnesiac: enhances AdCyc; dPACAP= pituitary AdCyc activating peptide Ddc: Dopamine decarboxylase rutabaga: defective Ca2+-calmod dep. AdCyc dunce: PDE mutation