Behavioral modulation induced by food odor aversive conditioning and its influence on the olfactory responses of an oscillatory brain network in the slug Limax marginatus.

We compared behaviorally and physiologically the olfactory responses of slugs (Limax marginatus) that had been subjected to aversive, appetitive, or unpaired training with food odors (carrot or cucumber). In the aversive training, the slugs were exposed to the food odor as a conditioned stimulus (CS), and then quinidine sulfate solution as an unconditioned stimulus (UCS) was immediately applied to the lip of the slugs. This training caused a decrease in preference level for the CS. The unpaired training, in which the CS and the UCS were presented to the slugs with a 5-min interval, induced no change in the preference level for the CS. In the appetitive training, the slugs were allowed to eat the CS odor source without UCS application. When we used nonstarved slugs, it was found that the preference level for the CS increased upon the appetitive training. These results indicate that each training changed the preference for the odors in a characteristic manner. In the physiological experiments, we used brain-inferior tentacular nose preparations isolated from slugs and investigated the olfactory responses of the oscillations in the local field potential (LFP) of the procerebral (PC) lobe. We found that odor presentation induced various types of changes in the LFP oscillation frequency, although the rate of occurrence of the frequency modulation differed between odors used in the aversive and the unpaired training (aversive-conditioned and unpaired odors). The aversive-conditioned odors induced a decrease in the oscillatory frequency. Unpaired odors did not change it. Moreover, odors used in the appetitive training (appetitive-conditioned odors) induced an increase in the frequency. Thus, it was considered that those modulations of PC lobe oscillatory activity were independent of odor and reflected learned preference for odors.