Biophysical mechanisms underlying the generation and maintenance of rule-learning engram

ABSTRACT Training rodents in a particularly difficult olfactory-discrimination task results with acquisition of high skill to perform the task superbly, termed ‘rule-learning’. We show that rule-learning occurs abruptly, in a “light bulb moment”. Using whole-cell patch-clamp recordings in the piriform cortex of Fos2A-iCreER/TRAP2 mice, we targeted activated neurons, expressing immediate early genes (IEG). From the onset of training, IEG-positive neurons from trained animals display enhanced intrinsic excitability. Subsequently, synaptic excitation and inhibition is enhanced in these neurons, in a coordinated, cell-wide process. In parallel, the density of IEG-expressing neurons sharply declines. Double labeling with TRAP and c-Fos reveals that nearly two-thirds of the rule-memory engram neurons were engaged from the beginning of training. Silencing TRAP-expressing neurons using DREADDs leads to a complete loss of rule memory. We propose that rule learning occurs at a discrete moment, and is developed through a gradual process that stabilizes the memory of the rule.