PSYC 310 Lecture Notes - Lecture 15: Creb-Binding Protein, Eyelid, Long-Term Depression
7 May 2018
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PSYC 318: Wednesday April 4th, 2018
Eye blink conditioning and LTD
• Learning objectives
o Mechanisms of cerebellar LTD
▪ Circuit underlying LTD
▪ PKC as an associative molecule
▪ How phosphorylation of GluA2 affects its trafficking
o What phosphorylation means and how this determines effect of mutating amino acid that
is phosphorylated
o How to test whether a form of cellular plasticity is important for behavior
• Behavior
o Pairing of the CS (tone) to the US (puff in the eye) allows for closure of eyelid to the tone
o An associative memory
o Puff in eye is aversive stimulus
o This is more specific than fear-conditioning
▪ Animal learns to be afraid when it hears the tone but it doesn’t have to do
anything specific to try and avoid the shock
▪ Testing if animal remembers in context of tone by showing fear
▪ Here, animal must do specific motor response in able to avoid the aversive
stimulus (close its eyes)
o Important for timing of response but not for the association
• Circuitry of eyeblink conditioning
o The tone comes through auditory nucleus (pontine) to cerebellum through mossy fibers
o Granule cells in cerebellum that synapse onto Purkinje neurons
o How the CS reaches the Purkinje neurons in Cerebellar cortex and output neurons in
interpositus nucleus (both in the cerebellum)
o US (puff to eye) comes through trigeminal nucleus to inferior olive to cerebellum by
climbing fibers, one single climbing fiber enough to depolarize
• Purkinje neurons
o Only found in cerebellar cortex in one place
o Fairly unique type of neuron, distinguishable from others based on appearance/gene
expression
o Neurons were determined to be locus for learning, if you ablated them the animal didn’t
perform eyeblink conditioning
o Purkinje neurons are beautiful
▪ Dendritic trees = Purkinje neurons (the term came from these neurons)
• Purkinje neurons receive 2 inputs
o Mossy fibers from pontine nucleus
▪ From CS, synapse on granule cells, onto Purkinje neurons through parallel fibers
representing CS
▪ We have insane # of granule cells in brain (about 50% of all neurons in brain are
cerebellar granule cells)
▪ Input to Purkinje neurons
o Climbing fibers (one input to P neurons) represents US and comes from inferior olive
o Association of the two: CS coming through parallel fibers and US from climbing fibers
decrease in synaptic strength, LTD of synaptic input
o Purkinje cells inhibitory (contain GABA)
▪ Prevent output neurons from firing
▪ Decreased firing P cells: increase firing of output neurons and increase in
eyeblink response
• LTD in P cells
o Pairing firing of climbing fibers w/ parallel fibers leads to decrease in synaptic strength in
parallel fiber input
o Climbing fibers prpobably represent depolarization while input is changed at parallel
fibers
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• Climbing fiber is an error signal
o The animal made a mistake, doesn’t want puff ot reach eye
o Coupling of climbing and parallel fibers depress the inputs that are in error: decreases
them
o Since they’re decreased, idea that P cells will fire less at the right time
• Evidence for this idea:
o Experiment measuring closing of eyes (eyeblink)
o
o Tone comes where black area starts (left side) right before puff
o Add inhibitor of GABA transmission/GABA receptors (picrotoxin) then animals blink eyes
as soon as they hear the tone
▪ Close eyes too early
o Animal learned:
▪ Blocking of P neurons doesn’t affect animal’s association of tone to puff
▪ But they have to close their eyes at the right time
▪ No P cell input hear tone and close eyes right away, then you tend to open
your eyes when puff comes and this doesn’t work out well for you
• How this could work:
o Couple of ideas about how P cells regulate output neurons
▪ Blue lines granule cells
▪ Inverter model: P cells stop firing at right time to close eyes, drives behavior
through output neurons
▪ Could also be that the actual stopping of inhibition causes rebound (T-type
rebound)
• P cells stop firing and you get ouput
▪ Some people think that what happens is P cells fire in synchrony, and in this
firing that’s a signal to cerebellar output neurons to fire (only when they’re firing
synchronously)
• P cells important for brain rhythms
o The first two models: LTD makes sense as a mechanism for the plasticity
o LTD won’t work with last model, synchrony coding
• How this could work…
o Inverter
▪ P cells don’t fire at the right time to blink, consistent with LTD mechanism
o T-type rebound
▪ CBN (cerebellar nucleus) fire when released from inhibition at right time to blink,
consistent with LTD mechanism
o Synchrony
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▪ CBN fire after synchronous inhibition at right time to blink, not consistent with
LTD mechanism
• Purkinje cells (I am abbreviating them as P cells) control timing
o First learning occurs in output part of cerebellar cortex
▪ Don’t need P neurons here
▪ Associates tones with closing eyes
o Timing requires P cells
▪ P cells repress this response, when they release output cells timing is right. P
cells fire during the delay
• Reduced LTD in culture
o Hard to study in intact brain
o Culture P neurons, mimic climbing fibers by depolarizing P neurons
o Mimic parallel fibers by puffing on transmitter used by parallel fiber glutamate
▪ Pair depolarization w/ parallel fiber input (glutamate depolarization) long term
depression of response of glutamate
▪ Mimic response with just isolated P neurons
o Glutamate not required fro activation of NMDARs, puff of glutamate required to activate
g-protein metabotropic receptor
▪ Depolarization important for calcium entry: get rid of depolarization if they
manipulated increase in calcium in other ways
• Associative molecules in cultures is PKC
o Protein kinase C
o Calcium entry from depolarization and DAG (diacylglycerol)
o PKC directly phosphorylates AMPA receptor
▪ GluA2
▪ Phosphorulation site present only on GluA2 not GluA1 specificity
▪ Reduces binding to protein called GRIP (with a PDZ domain)
▪ Retains AMPAR at synapse, differnet PDZ ptortein then binds (PICK) which
‘picks’ the receptor off membrane and causes it to endocytose
▪ Allows AMPAR to get uncoupled from where it’s working and get endocytosed
into the membrane
• Regulation of endocytosis
o
o End of GluA2 are amina acids SLKI
▪ Serine (S) gets phosphorylated
▪ L and I are recognition site for these proteins called PDZ proteins (have PDZ
domain) major determinants (leucine, isoleucine) important for binding of
GRIP and PICK
▪ GRIP and PICK differ by how much they care about serine (S)
• GRIP bound to begin with
o Doesn’t like phosphorylated S
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find more resources at oneclass.com
