Vagal nerve stimulation (VNS) is known to improve cognitive processing, presumably by affecting activity in central nervous system structures that process recently acquired information. It has long been assumed that these effects are related to stimulation-induced increases of norepinephrine (NE) release in limbic brain structures. The present study examined this hypothesis by administering VNS at an intensity and duration that improves memory and then measuring fluctuations in NE output in the basolateral amygdala (BLA) with in vivo microdialysis. In Experiment 1, VNS caused a 98% increase in NE output relative to baseline. In Experiment 2, methyl atropine was given 10 min before VNS to assess whether stimulation-induced increases in amygdala NE are mediated by afferent or efferent vagal branches. Methyl atropine did not alter NE release in the BLA in comparison with saline. The significance of these findings in understanding how peripheral neural activity modulates limbic structures to encode and store new information into memory is discussed.

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The experiments examined the effects of prelimbic-infralimbic inactivation in rats on the acquisition and reversal learning of different discrimination tasks: 2- or 4-choice odor discrimination in Experiments 1 and 2, the shift from 2-choice odor discrimination to 2-choice place discrimination in Experiment 3, and the shift from 2-choice place to 2-choice odor discrimination in Experiment 4. Infusions of 2% bupivacaine did not impair performance in the odor discrimination tests. Prelimbic-infralimbic inactivation did not impair acquisition but did impair the shift from an odor to a place discrimination and vice versa. Analysis of the errors revealed that the deficit was due to perseveration of the previously learned strategy. The selective deficits observed in the odor-place tests suggest that the prelimbic-infralimbic areas enable behavioral flexibility when conditions demand inhibiting the use of one type of attribute information and learning a new type of attribute information.

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