Learning and memory

The effects of genes and drugs

• Synaptic Self. Joseph LeDoux. Penguin, 2002, New York.– Excellent, enjoyable introduction to neurons,

brain, memory and behavior

Types of memory

• There is general agreement that there are several different types of memory, each of which is predominantly in a different part of the brain.

Declarative vs. procedural memory

• Declarative memory – explicit memory– facts, dates, events– Hippocampus is critical

• Procedural memory – non-declarative/implicit– how to perform an action (ride a bicycle)– Basal ganglia is critical

• Hippocampus is often first region to show damage in Alzheimer's disease

• Patients with Alzheimer's disease are unable to learn or remember ordinary facts (declarative memory) but are normal or nearly normal at learning and remembering how to do things (procedural memory).

Memory experiment

• Alzheimer's patients learned and remembered how to read complex words in a mirror as well as normal control subjects

• Were unable to recall the training session or the fact that they had acquired this skill.

Neurons

• Neurons are the cells that transmit information rapidly in the body.

• Provide mechanism for faster responses to the environment

Three main parts of neurons

• Dendrites: – receive signals from sensors and/or other

neurons

• Cell body: – maintains cell

• Axon: – sends signals to other neurons or muscles

Synapse

• Neurons communicate with each other at synapses using chemical neurotransmitters.

• Information usually flows across the synapse from the axon terminal (presynaptic) to the receiving neuron (postsynaptic)

• Neurotransmitter released from the pre-synaptic neuron is detected by receptors on the post-synaptic neuron

How drugs act on synapses

• Neurons communicate with each other at synapses using chemical neurotransmitters.

• This provides the basis for drugs (and poisons) to affect synaptic transmission.

• Drugs that have chemical properties similar in some way to those of neurotransmitters can act on synapses to alter behavior and thoughts (psychotropic or psychoactive drugs)

• Drugs that increase synaptic transmission are "agonists".

• Drugs that block or reduce synaptic transmission are "antagonists".

Synaptic Self. Joseph LeDoux

• One type of neurotransmitter receptor present in synapse is the NMDA receptor

• Involved in learning

• Consists of subunit 1 (NR1) and one of various NR2 subunits.

Genetic enhancement of learning and memory in mice.

Tang YP et al.

• NMDA receptors present in synapse of neurons involved in learning

• Consist of subunit 1 (NR1) and one of various NR2 subunits.

• Hypothesis: Increased expression of the NR2B subunit in mice will increase synaptic plasticity and improve memory

Methods

• Created several strains of transgenic mice that overexpressed NR2B subunit in the forebrain.

• The transgenic mice have normal growth, body weight, mating behavior, and no obvious neurologic deficits.

• Assays showed about twice as much NR2B protein in the cortex and hippocampus as in the wild type mice, and elevated levels in the amygdala.

Performed several tests of learning

• NR2B transgenic mice remembered previously-seen objects better than control mice (spent less time inspecting previously-seen object).

• In water maze, NR2B mice learned significantly faster than wild-type.

• Did same experiment with transgenic mice from two different genetic strains. Got same results for each strain.

• Did similar experiments using contextual and cued conditioning (Pavlovian conditioning). NR2B animals had significantly better memory at 1 hour, 1 day, and 10 days.

• Increased expression of NR2B protein improves memory in mice

Reversal of Age-related Learning Deficits and Brain Oxidative

Stress in Mice with Superoxide Dismutase/catalase Mimetics

Liu, Liu, Bi, Thompson

Background

• Loss of learning and memory function in aging mice is associated with increases in markers of brain oxidative stress.

• Learning ability and levels of protein oxidation in brain are negatively correlation.

Hypothesis

• Synthetic drugs (Superoxide Dismutase mimetics) that remove reactive oxygen species (ROS) are beneficial in reversing age-related learning deficits.

SOD mimetics from Eukarion: EUK-189 and

EUK-207.

Treat mice with SOD mimetics

• Assign female mice at 8 months old to 6 groups (16-18 per group)– control– untreated control– low dose EUK-189– high dose EUK-189– low dose EUK-207– high doseEUK-207

• Minipumps implanted in anesthetized mice.

• drug delivered for 28 days

• low rate ~9 nmol/day

• high rate ~ 90 nmol/day

• Pumps replaced twice of over 3 months of treatment.

Behavioral testing after 3 months

• Plexiglass cages.

• Videocam to record freezing behaviour (indcates of fear conditioning).

• Computer to control events.

• Mice placed alone in clean chamber.

Learning test

• Generate tone (e.g., 800 Hz), or context (particular cage)

• Give mild electric footshock

• Next day, generate same tone, or put mouse in context cage

• Measure fear conditioning as % of time mice exhibit a freezing response (absence of all movement except breathing)

Effects of SOD/catalase mimetic on fear conditioning learning

• control mice – 11 month old control mice show impaired

learning and memory: low levels of freezing for both tone and context

• mice treated with SOD mimetic – 11 month old mice treated with SOD mimetic

show superior memory: increased tone and context freezing at both doses

• Compared memory performance – 11 month old mice treated with SOD mimetic

versus control mice at 8 months old– Similar performance in memory tests– Drug appears to reverse impairment of older

mice

Effects of SOD/catalase mimetic on brain oxidative stress

• Mimetics decreased age-related free-radical damage: – decreased lipid peroxidation– decreased protein oxidation

Donepezil and flight simulator performance: Effects on retention

of complex skills

J.A. Yesavage et al.

• Donepezil is an FDA-approved drug used to treat Alzheimer’s disease.

• Can donepezil improve memory in healthy adults?

• Yesavage and colleagues performed a randomized, double-blind, parallel group, placebo controlled study to test the effects of the donepezil (5 mg/d for 30 days),

• Memory test: Ability to remember a sequence of complex tasks in flight simulator

• 18 licensed aircraft pilots, mean age of 52 years.

Results

• After 30 days of treatment, the donepezil group showed greater ability to retain the capacity to perform a set of complex simulator tasks than the placebo group, p <0.05.

• Donepezil appears to have beneficial effects on retention of training on complex aviation tasks in nondemented older adults.

• See also letter critiquing methodology and result.

The BDNF val66met polymorphism affects activity-dependent secretion of BDNF

and human memory and hippocampal function

Egan et al.

Cell, Vol. 112, 257–269, January 24, 2003,

Background

• Episodic memory – the ability to store and recall experiences and events – depends on the hippocampus.

BDNF

• Brain-derived neurotrophic factor (BDNF) appears to play an important role in memory formation.

• BDNF enhances synaptic transmission by facilitating synaptic vesicle docking.

• Inhibition of BDNF signaling in rodents by gene knockout or infusion of antisense BDNF impairs spatial learning and memory.

• Newly-synthesized BDNF protein is packaged into secretory vesicles.

• A polymorphism in the human BDNF gene has been identified.

• The polymorphism changes the amino acid at codon 66 from valine to methionine

• Refer to as val66met

• This polymorphism located in the 5' pro-BDNF-sequence, and thus is unlikely to alter the intrinsic biological activity of the mature protein.

• However, the authors hypothesized that this polymorphism would affect intracellular processing and secretion of BDNF, leading to impairments in hippocampal function in humans.

• They also hypothesized that BDNF genotype might be associated with genetic risk of schizophrenia.

Methods

• To test these hypotheses, the authors examined the effects of the BDNF val66met substitution in a cohort of normal controls, patients with schizophrenia and their unaffected siblings.

Effects of BDNF genotype on episodic memory

• Examined the effects of BDNF genotype on measures of episodic memory in a cohort of 621 subjects, including – normal controls– patients with schizophrenia, and – their unaffected siblings

• Evaluated memory using the Wechsler Memory Scale (WMS), a test of verbal episodic memory.

• Delayed recall scores on WMS reflect the amount of information from two stories with 50 total elements that subjects are able to recall following a 0.5 hour delay.

Results

• Patients with schizophrenia had substantially lower scores compared to controls

Results

• Looking at all three groups (normal, patients, siblings)– BDNF genotype had a significant effects on

memory scores (p=0.02).

• Looking just at the 133 controls alone, – BDNF genotype had a significant effects on

memory scores (p=0.008).

Val- and Met-BDNF expression in cultured hippocampal neurons

• Egan examined the expression, processing, biological activity, and release of the two BDNF isoforms.

• Expressed constructs containing either valBDNF or met BDNF attached to green fluorescent protein (GFP)

• valBDNF-GFP – distributed throughout the cell body and

extending to processes.

• metBDNF-GFP – mainly in cell bodies

• valBDNF-GFP– Neuronal activity-dependent secretion was

normal.

• metBDNF-GFP – Neuronal activity-dependent secretion was

severely reduced and sometimes not detectable.

• valBDNF-GFP-containing particles were often superimposed with punctuate staining for the synapse marker protein synaptophysin.

• metBDNF-GFP showed no colocalization with synaptophysin.

• metBDNF’s failure to localize at synapses may contribute to memory impairment and to schizophrenia.

Clinical caveats

• The authors point out several caveats with regard to this work.

• 1. The magnitude of the effects of BDNF genotype on in vivo measures of hippocampal function is small. This is expected, given the likely polygenic aspects of these traits.

• 2. Spurious results can arise when ethnic groups are mixed ("population stratification").

• Because the effects are found in a single ethnic group (European Americans) and because similar effects are seen in both normal controls in patients, the authors believe that stratification is unlikely to explain their results.

• 3. Genetic association per se cannot exclude a potential effect of another nearby SNP in linkage disequilibrium with val66met.

• The authors re-sequenced the BDNF coding region in 16 subjects (32 chromosomes) and scanned for polymorphisms in 66 subjects.

• Only one rare synonymous change was observed, suggesting that linkage disequilibrium to a nearby locus does not account for these results.

• The authors argue that their demonstration that the met allele is both associated with relatively poorer hippocampal function in vivo and with impaired trafficking and regulated secretion in vitro indicates strongly that the effects in humans are due specifically to the met allele.

What determines intelligence?

• Likely the cumulative effect of variations in large numbers of genes, plus environmental factors.

• Exposure to chemicals and drugs during fetal development can adversely affect neuron growth, formation of synapses.

• Future treatments may involve gene therapy to replace poorly-functioning genes.