Olfactory learning induces differential long-lasting changes in rat central olfactory pathways

In the present work, we investigated lasting changes induced by olfactory l earning at different levels of the olfactory pathways. For this, evoked fie ld potentials induced by electrical stimulation of the olfactory bulb were recorded simultaneously in the anterior piriform cortex, the posterior piri form cortex, the lateral entorhinal cortex and the dentate gyrus. The ampli tude of the evoked field potential's main component was measured in each si te before, immediately after, and 20 days after completion of associative l earning. Evoked field potential recordings were carried out under two exper imental conditions in the same animals: awake and anesthetized. In the lear ning task, rats were trained to associate electrical stimulation of one olf actory bulb electrode with the delivery of sucrose (positive reward), and s timulation of a second olfactory bulb electrode with the delivery of quinin e (negative reward). In this way, stimulation of the same olfactory bulb el ectrodes used for inducing field potentials served as a discriminative cue in the learning paradigm. The data showed that positively reinforced learni ng resulted in a lasting increase in evoked field potential amplitude restr icted to posterior piriform cortex and lateral entorhinal cortex. In contra st, negatively reinforced learning was mainly accompanied by a decrease in evoked field potential amplitude in the dentate gyrus. Moreover, the expres sion of these learning-related changes occurred to be modulated by the anim als arousal state. Indeed, the comparison between anesthetized versus awake animals showed that although globally similar, the changes were expressed earlier with respect to learning, under anesthesia than in the awake state. From these data we suggest that associative olfactory learning involves dif ferent neural circuits depending on the acquired value of the stimulus. Fur thermore, they show the existence of a functional dissociation between ante rior and posterior piriform cortex in mnesic processes, and stress the impo rtance of the animal's arousal state on the expression of learning-induced plasticity. (C) 2001 IBRO. Published by Elsevier Science Ltd. All rights re served.