CNS Synapses and long term plasticity

What are the types of memory?

• declarative - explicit

  • conscious aware, what where and when

• non declarative - implicit

  • not conscious, procedural

  • associative - priming

  • associative - simple classic conditioning

  • non associative - habituation, sensitisation

  • skills

Tests of declarative memory:

• word list, delay 15 seconds, try to recall

• object recognition - visual

• object placement - spatial

Location of declarative memory and what blood flow increases

• hippocampus - declarative memory

  • if the hippocampus is removed then there is declarative memory loss but not procedural

• increase in cerebral blood flow following object recall

how was the first observation of LTP conducted? terje lomo experiment

activation of perforant path input to dentate gyrus

1. synaptic transmission monitored at <0.1Hz - low frequency stimulation of perforant path

2. brief high frequency 100Hz stimulus applied to same input - tetanus

3. return to LFS (<0.1Hz)

outcomes from the terje lomo experiment?

• steeper rise time of the EPSP

• increase in number of cells firing action potentials

  • lasts for several hours

Which synapses is LTP seen for in vitro?

LTP at CA3→ CA1 synapses

Experimental set up for LTP at CA3 → CA1 synapses

examples of 2 types of experiment to demonstrate LTP is complex - early and late

• with single 100hz there is only early LTP which lasts shorter

• with four 100Hz there is early and late LTP which lasts longer

What are the component phases of LTP

1. induction phase - initiation of LTP by HFS 100Hz stimulus

2. transient or early - reversible early LTP following induction

3. consolidated or late - permanent changes which maintain late LTP

what is induction process mediated by

NMDA receptors

AP5

• NMDA receptor agonist

• blocks LTP and LTD during HFS

induction process

• postsynaptic rise intracellular calcium

• postsynaptic depolarisation

• mediated by NMDA receptors

what leads to baseline baseline neurotransmission at LFS

• glutamatergic transmission at -80mV

• only AMPA receptors are responsible for baseline neurotransmission

does D-AP5 have effect on EPSPs evoked by <0.1Hz

no

what happens during 100Hz stimulation for induction of LTP

• temporal summation from activation of pre synaptic terminals, sustained activation, release of neurotransmitter

• EPSP at -40mV

• After AMPA activation there will be NMDA activation

  • requires more depolarised membrane potential

Evidence for NMDA-R in declarative memory

• morris water maze - for spatial memory

• AP5 treated mouse still cannot find the platform

Role of protein kinases during early LTP

• No block seen when AP5 applied after induction - shows NMDA receptor activation is independent

• dependent upon activation of protein kinase C and or calmodulin dependent protein kinase II

• increased phosphorylation of AMPA receptor - both by PKC and CaMKII

what does increased phosphorylation by PKC and CaMKII lead to

• allows increased conductance of AMPA receptor ion channels

• inc current flow inc EPSC amplitude

• EPSP in larger

Role of protein kinases during late LTP

• protein kinase A dependent

  • seen through inhibition of H89. therefore involves increased cAMP

• anisomycin - transitional inhibitor

• actinomycin d - transcriptional inhibitor

  • DNA is targeted by activation of PKA

• anisomycin

• transitional inhibitor

actinomycin d

transcriptional inhibitor

if it is only LTP what happens when all the synapses have undergone LTP

• eventually all the synapses would become fully potentiated

• ceiling point with no inc in synaptic efficiency

• therefore LTD reverses and depresses LTP

experimental setup for LTD in CA3 → CA1 synapses

• same at LTP

• but only 1Hz - low frequency stimulation instead of 100Hz high frequency

How is LTD and LTP similar mechanistically?

• both blocked by AP5

• both depend on NMDA-R activation

• postsynaptic membrane potential depolarisation

• increase in postsynaptic intracellular calcium

How is LTD mechanism different to LTP?

• not blocked by protein kinase inhibitors

• expression is blocked by FK506 and okadaic acid

  • protein phosphatase inhibitors instead

Difference in activation threshold for LTD and LTP?

• difference in activation threshold for protein phosphatases vs kinase

intracellular calcium determines which is induced

• <1μM → LTD (dephosphorylation).

• 5μM → LTP (phosphorylation).