Genetic dissociation of acquisition and memory strength in the heat-box spatial learning paradigm in Drosophila

Memories can have different strengths, largely dependent on the intensityof reinforcers encountered. The relationship between reinforcement and memorystrength is evident in asymptotic memory curves, with the level of theasymptote related to the intensity of the reinforcer. Although this is likelya fundamental property of memory formation, relatively little is known of howmemory strength is determined. Memory performance at different levels in Drosophila can be measured in an operant heat-box conditioningparadigm. In this spatial learning paradigm, flies learn and remember to avoidone-half of a dark chamber associated with a temperature outside of thepreferred range. The reinforcement temperature has a strong effect on thelevel of learning in wild-type flies, with higher temperatures inducingstronger memories. Additionally, two mutations alter memory-acquisitioncurves, either changing acquisition rate or asymptotic memory level. The rutabaga mutation, affecting a type-1 adenylyl cyclase, decreases theacquisition rate. In contrast, the white mutation, modifying an ABCtransporter, limits asymptotic memory. The white mutation does notnegatively affect classical olfactory conditioning but actually improvesperformance at low reinforcement levels. Thus, memory acquisition/memorystrength and classical olfactory/operant spatial memories can be geneticallydissociated. A conceptual model of operant conditioning and the levels atwhich rutabaga and white influence conditioning isproposed.