Eyeblink Conditioning: From Reflex to Consciousness

PSY391S

April 3, 2006

John Yeomans

Pavlov and Search for Engram

• Visceral reflexes: Salivation and gastric acid.

• Laws of conditioning: pairing, extinction, recovery, generalization, etc.

• Conditioning in Cortex?• Search for Engram: Lesions of cortex don’t

block learning of mazes or conditioning (Lashley).

• Correlates of learning: whole cortex active initially.

Eyeblink Conditioning

• Easier to measure in rodents and humans.• Slow acquisition and extinction.• Disynaptic reflex circuit for unconditioned

reflex (US-shock and UR) in brain stem.• Activity in hippocampus and cerebellum

correlates with acquisition of delay conditioning.

• Hippocampus not critical; ipsilateral cerebellum is!

Recording from Cerebellum

• Activity in Interpositus or Red N. precedes and predicts conditioned response (CR).

• Microlesions or inhibition of Interpositus or Red N. blocks learning (Thompson).

• Circuits for CS (tone), US (shock) found in CBel.• Purkinje cells inhibited by pairing climbing fiber

and parallel fiber stimulation: Long-term depression.

• Similar for leg flexion and vestibular-ocular reflex (Ito)

Interpositus activityGreater Eyeblink

CS and US PathwaysTo Cerebellum

Trace Conditioning

• Gap between CS and US. Harder to learn.

• Hippocampus needed for trace conditioning, but not delay conditioning.

• Blocked by MAM, a poison that prevents neurogenesis in dentate gyrus.

• MAM does not block fear conditioning, but that is easier task.

Awareness

• Eye-blink conditioning in humans.• Hippocampus damage blocks trace conditioning,

but not delay conditioning.• When asked later, normal subjects can say

whether CS and US were paired for trace task, but not for delay task (Clarke and Squire).

• Awareness related to success of trace conditioning, but not delay conditioning.

• Hippocampus stimulation.• Search for Consciousness—Imaging correlates

and testing awareness?

Plasticity and Learning

PSY391

April 5, 2006

John Yeomans

Neurons and Learning

• Simple circuit approach—Aplysia• Monosynaptic reflex.• 7 giant Motoneurons identifiable.• 30 sensory neurons identified.• Habituation, sensitization, conditioning.• Short term and long-term changes.• Synaptic changes, proteins and genes.• Kandel.

Sensitization:3 facilitating interneurons5HT increases release in presynatic terminalsLarger EPSP in Motoneuron L7

Mechanisms of Plasticity

• Habituation leads to smaller EPSP; Sensitization leads to larger EPSP.

• Changes in presynaptic terminal lead to more or less transmitter release (Ca++).

• Sensitization involves more cAMP, protein Kinase A, and K+ channel changes.

• Long term changes require gene transcription protein synthesis and CREB.

• Is this the same as mammals?

Hippocampus

• Slices allow intracellular study of neurons and synapses.

• Hippocampus is needed for new long-term declarative memories in humans.

• LTP plasticity has many properties of memory.

• Problem: Circuits into and out of hippocampus aren’t known, so the functions of neurons aren’t known.

Long-Term Potentiation

• Three glutamate synapses in series, dentate gyrus, CA3, CA1.

• All show LTP with high-frequency stimulation (100 Hz “tetanus”).

• LTP lasts for hours (early phase), days or weeks (late phase).

• Input specific, and associative.

• Like learning and memory?

LTP Mechanisms in CA1• AMPA depolarizes postsynaptic neuron to

remove Mg++.• Glutamate can open NMDA receptors.• Hi Ca++ entry activates CaMKII and PKC.• More AMPA receptors are added to

postsynaptic membrane early LTP (hours).

• In addition, NO can increase presynaptic release in some synapses (“retrograde transmission”). cGMPCa++ channels

Nucleus

NO made in Synapse not nucleus

LTD Mechanisms in CA1

• Low frequency stimulation (1-5 Hz).

• Low Ca++ activates phosphatases.

• Internalization of AMPA receptors Long-Term Depression.

Early and Late-phase LTP

• Early phase LTP (hours) does not require new protein synthesis (gene transcription).

• Gene transcription is needed for long-term LTP (days).

• Several kinases activate CREB, which activates gene transcription. Many signals (e.g. Ach, DA, NE, opiates) influence many kinases.

• Many proteins are needed for growth of dendritic spines and synapses for long-term changes.

Long-term Memories

PSY391S

April 10, 2006

John Yeomans

Short and Long-term Memory

• Retrograde amnesia after concussion. Memories return in order toward time of injury.

• Electroconvulsive shock induces RA similarly in humans and animals.

• Consolidation Hypothesis.• Protein synthesis inhibitors block long-

term storage of memories, but not STM, in animals.

Hippocampal Damage

• H.M. can’t form new, stable verbal (declarative) memories.

• He can form immediate memories (for seconds), but they are lost when distracted.

• He can learn new motor tasks (procedural learning). (Cerebellum and striatum, e.g.)

• He has high IQ and remembers events before surgery well.

Long-term Storage of Memories

• Hippocampus needed for laying down new LTMs, but not for long-term storage after weeks.

• Permanent memories and abilities are believed to be stored in cortical areas for each function, e.g. speech, personal history, complex skills, feelings.

Hippocampus in Rodents

• Needed for spatial memories: 8-arm-maze, water maze, Barnes maze.

• Needed for contextual fear conditioning, but not simple fear conditioning.

• Needed for trace conditioning but not delay conditioning.

• Needed for social communication between rats.• Long and short term memories different: Protein

synthesis needed for LTM and LTP.

Spatial Memory in Rats

How are memories converted to long-term, then to short-term forms?

• Theory: Synaptic changes are the basis of all memories.

• Number of synapses depends on dendrites and spines.

• Many proteins are needed to make synapses grow and retract.

• Dendrites and spines grow and retract.

Dendrite Growth

Spine Growth

Neurogenesis

• New neurons are formed in dentate gyrus and olfactory bulb (BRDU, 3H-thymidine markers).

• Needed for new olfactory memories, and for trace conditioning.

• Can be stimulated by serotonin, estrogen, seizures or genes.

• Can be inhibited by stress/depression and hormones, or by toxins (MAM, radiation).

Reconsolidation

• If memories are recalled again (new tests in rodents), they become more vulnerable to ECS or to protein synthesis inhibition.

• Are memories then reconsolidated in hippocampus?• Suggests transfer back and forth between more

permanent (cortex) and less permanent (hippocampus) forms.

• Limbic frontal cortex connected and active in these exchanges.

• How are memories recalled and brought back into temporary storage?

Long-term Storage

• How does hippocampus receive new information for memories? (via entorhinal cortex)?

• How does hippocampus convert memories into long-term stores? (frontal cortex, e.g. anterior cingulate)?

• How are long-term memories stored in cortex synapses?

• Are long-term stores lost in reconsolidation, and if so, how?

• How are memories exchanged between HPC, frontal cortex?

Genes and Memory

PSY391S

April 12, 2006

John Yeomans

Gene Control

• Knockdown of RNA: Antisense oligos (DNA) to inhibit mRNA in vivo.

• Knockout of Gene: Remove gene permanently from genome.

• Transgenic: Add extra copies of gene permenently.

• Inducible: Add promoter so that you canturn the gene on or off at will (tetracycline—Tet).

• Gene transfection by virus, electroporation, or inhibition by repressors.

Long-term Memories and CREB

• Long term memories improved by spaced trials vs. massed trials.

• Aplysia: CREB knockdown blocks long-term, but not short-term, sensitization.

• Block of Long-Term Memory (several tasks) and long-phase LTP in CREB knockout mice. STM and short-phase LTP unaffected.

Genes and Fruit Flies

• Olfactory memory can be tested in test tubes full of flies.

• Flies go toward smell, but shocked at one end of tube.

• Smart flies avoid, but dumb flies return, to end where shock given.

• Rutabaga, dunce, turnip all mutants that indicate that cAMP important for learning.

CREB

• CREB repressor before training blocks olfactory memory in flies, on second day, but not first day.

• Increasing CREB (by activator) leads to much improved long-term memory. One trial only needed for olfactory memory on next day.

• “Genius fruit flies”?

Improved Memories with NMDA and CREB

• Viral CREB in basolateral amygdala improves long-term, but not short-term fear-memories

• Viral CREB in VTA or N. Acc improves drug sensitivity.

• Memory improvement with stimulants, or added AMPA or NMDA receptors.

• Doogie: NR2B improves LTP and LTM

Viral-CREB in Amygdala

3 days 14 days

Alzheimer’s Disease

• Poor memory (senile dementia) + neural changes post mortem (plaques and tangles).

• B-amyloid and tau proteins.• Early onset due to APP and presenilins.• Down’s, APP and Ch21.• Late onset due to environment and to

ApoE eta4 copies.• Prediction of susceptibility by age and

genes.

Amyloid Plaques and Neurofibrillary Tangles

Dying of cholinergic axon terminalstauamyloid?

Genes and Alzheimer’s Disease

Amyloid Precursor Protein Ch21Presenilins Ch1

Apolipotropin e4 Ch19

Can amyloid production be slowed, stopped or reversed?

Can Alzheimer’s be stopped or reversed?

• Environment—Active lives, active brains.

• Cholinergic agonists. Slight slowing of loss.

• NGF? Anti-amyloid? Anti-tau?

• How long can we live productively and independently?

• Can we enhance memory?

• Should we enhance humans?