9.031 Neural Basis of Learning and Memory: Lecture 4

Classical conditioning in Drosophila: Cellular mechanisms

Flies demonstrate many forms of simple learning

• Flies have a proboscis extension reflex to sucrose which can non-associatively (habituated and sensitized) conditioned.
• Flies can learn discriminative conditioning tasks (positive and negative cues must be identified) in which visual, olfactory, and tactile cues are paired with positive (sucrose) or negative (shock) reinforcement.

The procedure involved a T-maze (two-arm choice apparatus). Separate odors (3-octanol (OCT) or 4-methylcyclohexanol (MCH)) could be delivered to each arm.

Flies were drawn into a training tube containing one of the odors, and electrically shocked with brief voltage pulses (10V to 150V).

Flies were then placed at the intersection of the T-maze containing both odors. The fraction of flies avoiding the previously shock-paired odor was the indicator of learning.

Different memory time course dependent on the training schedule

• After single training sessions, memory retention lasts about one day
• Multiple massed training sessions (10 sessions in immediate succession) gives four day retention (anesthesia-resistant intermediate term memory).
• Multiple spaced sessions (10 sessions with 15 minute inter-session intervals) gives greater than seven day retention (long-term memory).

So now that we have trained flies, what can we do with them?

rutabaga - mutation in the gene encoding Ca++/Calmodulin-dependent adenylate cyclase leading to lower cAMP levels. Moderate impairment of learning. Acquisition affected.

dunce - blocked cAMP phosphodiesterase which leads to high levels of cAMP. Moderate impairment of learning. Acquisition affected.

amnesiac - normal acquisition but deficient intermediate-term memory, affects a gene encoding a neuropeptide which acts through adenylate cyclase to increase cAMP. Suggests a role of neuropeptides in modulating or extending the effects of second messenger systems.

radish - normal long-term memory, disrupted anesthesia-resistant memory (intermediate term). Cellular mechanism is unknown.

dCREB2-b - blocks long-term memory, but acquisition and anesthesia-resistant memory (intermediate term) unaffected. Blockade of cAMP response element binding protein (CREB) transcription.

Memory stages can be dissected using pharmacological manipulations such as the protein synthesis inhibitor cycloheximide (CXM)

• Intermediate term, anesthesia-resistant component is CXM-insensitive, not dependent on protein synthesis or regulation of gene transcription.
• A longer term component is dependent on gene transcription and protein synthesis and is affected by CXM. Flies given CXM receiving spaced training show no seven-day retention.

• Acquisition - dependent on intact neurotransmitter and second messenger systems.
• Short/Intermediate term memory - dependent on direct, transient effects of second messenger activation such as protein phosphorylation or protein synthesis.
• Long term memory - dependent on gene transcription regulated by second messenger activation.

• Control time and duration of expression - the use of heat shock promoters in the dCREB2 mutant avoids developmental complications.
• Control tissue specificity of expression - mushroom bodies are implicated in insect learning and memory.
• Find similar vertebrate mutations. CREB mutant mice show long term memory deficits.