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LECTURE 20-21: CELLULAR BASIS OF LEARNING & MEMORY
REQUIRED READING: Kandel text, Chapter 63, and Assigned Review Articles
Research on cellular basis of learning & memory mainly performed in three animal systems
Aplysia
All neurons and synapses
in behavioral circuits are
identified and can be recorded
easily
Ideal for detailing mechanisms
underlying implicit learned motor
responses
Drosophila
Capable of
learned behaviors
Amenable to random
mutagenesis and
selection of mutants
with defective
behaviors
Mouse
Similar anatomy to human
Amenable to study of
explicit memory
Hippocampus amenable
to electrophysiology
Behavior modification of
genetically modified mice
APLYSIA SHORT-TERM LEARNED RESPONSES AFFECTING GILL WITHDRAWL REFLEX
HABITUATION
Repeated tactile stimulation of
siphon
depresses
gill withdrawl response
SENSITIZATION
Harmful stimulus
sensitizes
gill withdrawl response
to subsequent
harmful OR harmless
stimuli given to
same OR different
body regions
CLASSICAL
CONDITIONING
Pairing harmful stimulus
with preceding harmless
conditioning stimulus sensitizes
gill withdrawl response
to subsequent
conditioning stimulus
but not to tactile stimuli
given to other body areas
HABITUATION IS DUE TO DEPRESSED NEUROTRANSMITTER RELEASE AT SEVERAL SITES
Rapidly repeated tactile stimulation of siphon
attenuates gill withdrawl both during the
training and for a short period afterwards.
Habituation is due to reduced
neurotransmitter release by the
sensory neuron and by relevant interneurons
in response to the tactile stimulus.
I.e., the memory of habituation
is distributed at various synapses
in the circuit
Whereas a rapid series of stimuli induces
short-term habituation,
several sets of tactile stimuli distributed
over several hours induces
long-term habituation that lasts for weeks.
Long-term habituation requires
new protein synthesis and is due to
pruning of synaptic connections
SHORT-TERM SENSITIZATION IS MEDIATED THROUGH AXO-AXONIC
SEROTONERGIC SYNAPSES OF FACILITATING INTERNEURONS
Serotonergic facilitating interneurons
send axo-axonic connections to
broadly distributed sensory neurons
Unconditioned stimulus causes
interneurons to release serotonin,
which acts through metabotropic
HT receptors to increase the
likelihood of neurotransmitter release
following sensory neuron firing
Sensitization can be mimicked without
sensitizing stimulus by local
experimental application of serotonin
Sensitization is mediated by
presynaptic elevation of
cAMP & PKA activity,
which has three effects:
1)
2)
Greater proportion of vesicles
in active zone
(synapsin phosphorylation?)
K+ channel inactivation increases
duration of depolarization and
magnitude of Ca+2 influx
3) Activation of L--type calcium channels
CLASSICAL CONDITIONING EMPLOYS SEQUENCE-REINFORCED PRODUCTION OF cAMP
Conditioning is only effective when CS precedes US by a short interval (~ 0.5 sec)
CS elevates calcium in presynaptic terminal at moment of US.
Calcium/CAM enhances the enzymatic activity of adenylate cyclase triggered by 5-HT.
Adenylate cyclase is a biochemical “coincidence detector”
TEMPORALLY SPACED SENSITIZATION OR CONDITIONING TRAININGS
INDUCE LONG-TERM IMPLICIT MEMORY
Long-term sensitization
and conditioning are
also mediated through
presynaptic cAMP production
and PKA activity
PKA induces specific
CREB-dependent
gene transcription and
protein synthesis:
Newly synthesized ubiquitin
hydrolase degrades
PKA regulatory subunits,
making the enzyme
constitutively active
Other newly synthesized
proteins help build new
presynaptic terminals
onto motor neurons
GENETIC SCREENS FOR GENES AFFECTING CONDITIONING IMPLICIT MEMORY
ALL AFFECT THE cAMP-PKA-CREB PATHWAY
FLY MUTANTS SELECTED FOR DEFECTS IN IMPLICIT MEMORY
DUNCE
encodes cAMP phosphodiesterase
RUTABAGA
mutant defective for Ca+2/CAM enhancement of cyclase
AMNESIAC
encodes a peptide neurotransmitter acting on GS-coupled receptor
PKA-R1
encodes PKA
HIPPOCAMPAL NEURONS IN DIFFERENT RELAYS ARE ALL
CAPABLE OF UNDERGOING SYNAPTIC LONG-TERM POTENTIATION
One Theta burst gives what is sometimes called
Early LTP,
which is less than doubling
of EPSC which lasts for hours
Four Theta bursts spaced minutes apart generate
Late LTP,
with up to 4-fold EPSC stimulation
that lasts for days
AMPLITUDE OF EPSCS
20 min
1m
“THETA” BURST
60 min
EPSP Slope (% original)
AXON STIMULATION PROTOCOL
300
200
100
20
40
TIME (min)
60
80
INDUCTION AND EXPRESSION OF SYNAPTIC PLASTICITY
Prior synaptic activity can INDUCE long-term plasticity. Such plasticity can be INDUCED by
molecular events occuring either presynaptically or postsynaptically.
The changes in transmission following synaptic plasticity can be EXPRESSED either
presynaptically and/or postsynaptically, and need not correspond to the site of INDUCTION.
E.g., at a certain synapse, postsynaptic calcium influx can INDUCE plasticity which is then
EXPRESSED as changes in presynaptic neurotransmitter release probability.
LTP AT MOSSY FIBER--CA3 SYNAPSES IS DUE TO PRESYNAPTIC CALCIUM INFLUX
AND cAMP/PKA PATHWAY
LTP AT SCHAFFER COLLATERAL--CA1 SYNAPSES IS DUE TO
POSTSYNAPTIC CALCIUM INFLUX AND CAM KINASE ACTIVITY
LTP at CA3-CA1 synapse is blocked by
NMDAR antagonist APV and by inhibitors
of CAM kinase
PRESYNAPTIC COMPONENT OF EARLY AND LATE LTP AT
CA3--CA1 SYNAPSES RESEMBLES SHORT- AND LONG-TERM SENSITIZATION
Late LTP
absolutely requires
new protein synthesis
PRESYNAPTIC COMPONENT OF EARLY AND LATE LTP REQUIRES
POSTSYNAPTIC CAMK ACTIVITY AND RETROGRADE SIGNALS
OTHER MECHANISMS OF PLASTICITY ENHANCING EPSPS
LTP can be expressed postsynaptically as a reduction of leak conductance in dendritic spine.
This enables the EPSC to generate EPSP with greater length and time constants.
Excitatory transmission can be enhanced by HETEROSYNAPTIC INHIBITION OF INHIBITORY
TRANSMISSION. This is mediated by endogenous cannabinoids acting on presynpatic
terminals of nearby GABAergic synapses.
IS LTP REQUIRED FOR HIPPOCAMPAL CONSOLIDATION OF EXPLICIT MEMORY?
CAMK AND NMDAR1 NEEDED FOR LONG-TERM SPATIAL REPRESENTATION IN HIPPOCAMPUS
Single pyramidal neuron
in hippocampus
fires when mouse is in
certain location
(independent of
animal’s orientation)
Normal mouse remembers
where it has been.
spatial map in HC
does not change in
subsequent chamber trials
Mice with hippocampusrestricted mutations
in CAMK or NMDAR1
establish place fields,
but do not remember
from day to day
IS LTP REQUIRED FOR HIPPOCAMPAL CONSOLIDATION OF EXPLICIT MEMORY?
HIPPOCAMPAL CAMK AND NMDAR1 NEEDED FOR BOTH LTP AND SPATIAL MEMORY
SYNAPSES SENSITIVE TO NMDAR-MEDIATED LTP ARE ALSO SENSITIVE
TO NMDAR-MEDIATED LONG-TERM DEPRESSION (LTD)
AMPLITUDE OF EPSCS
20 min
1m
60 min
EPSP Slope (% original)
AXON STIMULATION PROTOCOL
300
LTP
200
100
20
40
60
80
20 min
5m
60 min
EPSP Slope (% original)
TIME (min)
300
200
LTD
100
20
40
TIME (min)
60
80
LTD HAS A LOWER CALCIUM CONCENTRATION THRESHOLD THAN LTP,
BUT LTP IS DOMINANT
LOW-FREQUENCY STIMULUS TRAIN
THETA- OR HIGH-FREQUENCY STIMULUS TRAIN
LOW-LEVEL CALCIUM ENTRY
GREATER CALCIUM ENTRY
ACTIVATION OF CALCINEURIN
ACTIVATION OF CALCINEURIN AND CAMK
AMPA RECEPTOR INTERNALIZATION
AMPA RECEPTOR INSERTION AND PHOSPHORYLATION
LTD
LTP
STRUCTURAL AND FUNCTIONAL FEATURES OF AMPA-TYPE GLUTAMATE RECEPTORS
AMPA receptors are homo- or hetero-tetramers
Restriction of calcium entry mediated by GluR2; tetramers containing >1 GluR2 subunit conduct only Na+/K+
AMPA receptors encoded by different genes or by alternative splicing have different C-terminal tails.
Receptor tails contain phosphorylation sites for different protein kinases and binding sites
for PDZ-domain-containing proteins
Receptors containing only GluR2(short) and/or GluR3 subunits are delivered constitutively from
vesicles to synapse
Retention at synapse mediated by complex with Glutamate Receptor Interacting Protein (GRIP)
Receptors containing at least one GluR1(long) subunit are stored in intracellular vesicles near synapse
During LTP, GluR1-containing tetramers are added to the synapse
NMDAR-INDUCED CAMK ACTIVITY ACTS ON AMPA RECEPTORS IN TWO WAYS
TO PROMOTE LTP
CAMK phosphorylates an unknown
protein, enabling a PDZ-protein
that interacts with long tail
on GluR1 to deliver receptor
TO EXTRASYNAPTIC SITE
Delivered receptors migrate (randomly?)
into post-synaptic density,
where interactions of receptorassociated GRIP and STG and the
major postsynaptic matrix protein
PSD-95 anchor receptor to synapse
Newly delivered GluR1-containing
AMPA receptors can be phosphorylated
directly by CAMK, which
increases unitary conductance
of the receptor
STG
GRIP
PSD-95
GRIP
PSD-95
CAMK
PDZ-protein
Calcineurin
Calcineurin activation promotes internalization
of AMPA receptors containing only
short-tail subunits, thereby promoting LTD
WHEN HIGH CALCIUM ENTRY ACTIVATES BOTH CALCINEURIN AND CAMK,
CAMK-MEDIATED GluR1-CONTAINING AMPAR EXOCYTOSIS EXCEEDS CALCINEURIN-MEDIATED
SHORT TAIL-ONLY AMPAR ENDOCYTOSIS
HIGH CAMK ACTIVITY INDUCED DURING LATE LTP IS ALSO MEDIATED BY
NEW CAM KINASE PROTEIN SYNTHESIS NEAR THE SYNAPSE
Most mRNAs have 3’ polyA tail, which is necessary for initiation of the mRNA’s translation
Neurons contain some mRNAs that are not polyadenylated, are not translated,
and are transported along dendrites to areas near dendritic spines
NMDA receptor activation and calcium entry activates a protein kinase
called AURORA
Aurora kinase activates translation of nearby dormant mRNAs
ONE OF THESE DORMANT RNAs ENCODES CAM KINASE
Because of its dendritic localizaation, new CAMK synthesis is restricted to the synapse undergoing LTP
The dendritic localization of dormant CAMK RNA and its activation during LTP are mediated by
Cytoplasmic Polyadenylation Element Binding (CPEB) protein
HOW DOES CPEB PROTEIN CONTROL RNA DORMANCY AND ACTIVATION IN NEURONS?
PolyA is needed for assembly of 5’
translation initiation complex
CPEB protein binding to 3’ CPE
helps mask RNA 5’ end
CPEB phosphorylation by Aurora allows for recruitment of polyA polyermerase (PAP)
Polyadenylation of dormant RNA allows assembly of 5’ translation initiation complex