SAPOLSKY Ch' 10 STRESS AND MEMORY

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SAPOLSKY Ch. 10STRESS AND MEMORY

• (Additional material)

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Nonlinear aspect of glutamate synapses

• 2 types of ionotropic (ion channel) glutamate
  receptors
• AMPA depolarizes by letting Na flow in.
• NMDA ion channel is normally blocked by a
  magnesium (Mg2) ion, which is attracted to the
  inside by the negative resting potential.

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With strong depolarization by sodium (Na)
through AMPA receptors, Mg2 is repelled. Ca2
comes in causes more AMPA receptors to insert
into membrane
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The additional AMPA receptors make the neuron
more responsive to future glutamate signals.
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Effect of short term moderate CGs

• Glucocorticoids enhance responsiveness of
  hippocampal neurons.
• One mechanism for remembering emotional events.

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Effects of SNS

• Arouse hippocampus
• Increase glucose
• Increase delivery of oxygen glucose to brain

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Effects of severe or prolonged stress

• GCs kill hippocampal neurons.
• (vicious circle less inhibitory control of GCs)
• Dendrite and axon retraction.
• Inhibition of hippocampal neurogenesis.
• Long-term depression (opposite of long term
  potentiation, which is a basis for memory
  formation)
• Uses a different glutamate receptor metabotropic
  receptor (mGluR) slow rates of stimulation.
• AMPA receptors are REMOVED from synapse.

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Stress early in life

• Impairs memory in middle-aged animals (and
  people)
• Michael Meaneys High-licking--grooming mothers
  counteracted effects of stress.

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Summary

• Three time frames for memory
• Sensory
• Short-term
• Long-term.
• Two main types of memory
• Explicit (factual)
• Requires hippocampus cortex
• Implicit (procedural, motor)
• Requires cerebellum

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Summary

• HM severe loss of explicit memory.
• Due to surgical removal of hippocampus
• Implicit memory intact
• Glutamate synapses are nonlinear.
• A little glutamate doesnt do much because NMDA
  receptors are blocked by Mg2.
• Strong depolarization repels Mg2 and Ca2 can
  come in via NMDA receptors.
• Ca2 results in installation of AMPA receptors in
  membrane ? greater sensitivity to glutamate.

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Summary

• Facilitative effects of moderate short-term
  stress
• Increased GCs ? increase hippocampus
  responsiveness
• SNS ? arouses hippocampus
• SNS increases glucose
• SNS increases delivery of glucose to brain

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Summary

• Severe or long-term stress
• Long-term depression (LTD) via mGluRs
• Retraction of dendrites axons
• Decreased neurogenesis
• Increases cell death in hippocampus
• Early stress ? memory loss in middle age
• (Michael Meaney mothers make a difference)

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Summary

• High GC (corticosterone in rats) ? deficits in
  recalling already-formed memories
• Cushings syndrome, PTSD, and GCs (used to treat
  autoimmune diseases) decrease size of hippocampus
  and impair memory.