Neural mechanisms of odor rule learning.

Rats that are trained in a particularly difficult olfactory discrimination task demonstrate a dramatic increase in their capability to acquire memories of new odors once they have learned the first discrimination task. Such high-skill learning, termed "rule learning" or "learning set" (see Saar et al., 1998, 2001) is accompanied by a series postsynaptic cellular modifications which have three major traits: a.They are widespread throughout the piriform cortex network. Both physiological and morphological modifications are found in most of the studied neurons.b.The time course in which these modifications appear and disappear is strongly correlated with the time course in which the skill is acquired and decayed. However, memories for specific odors outlast these modifications by far. Thus, the identified modifications are related to rule learning (learning how to learn), rather than to long-term memory for the specific odors for which the rats are trained.c.While the above-mentioned changes act to enhance single-cell excitability, others reduced it; synaptic inhibition is enhanced after learning and the subunit composition of the NMDA receptor is modified in a manner that favors activity-induced synaptic weakening over synaptic enhancement.