Effects of excitotoxic brain lesions on taste-mediated odor learning in the rat

The association learning between taste and odor is important in ingestive behavior. For a better understanding of this learning, we have developed a convenient and useful paradigm to assess the taste-mediated odor learning. In the training session, Wistar male rats drank water from two bottles in their home cages and from eight small glass dishes. In the learning session they were exposed in their home cages and also in a circular open-field apparatus to 0.005 M Na-saccharin and 0.02 M quinine hydrochloride which contained either banana or almond odors. One learning trial consisted of this pair of exposures. The preceding behavioral experiment has shown that these two odors are not aversive and are differentially perceived by rats. In the test session, the animals were put in the open-field apparatus equipped with eight dishes: four contained water with banana, and another four, with almond. Normal control rats preferred to drink water with the odor previously associated with saccharin. Stronger and more persistent preference was attained after two or three learning trials. To elucidate the brain sites responsible for this taste-mediated odor learning, the same procedure was assessed on brain-lesioned rats. Rats with lesions in the amygdala showed rapid extinction of preference to the saccharin-associated odor, whereas control rats did not. However, rats with lesions in the insular cortex showed retention of learning similar to that of the control rats. Rats with lesions in the sulcal prefrontal or cingulate cortices showed moderate disruptive effects on preference to the saccharin-associated odor. In conclusion, the odor learning established in our experimental paradigm is based on the association between the quality of odor and hedonics of taste. The amygdala may play a role in the formation, at least in the retention process, of this taste-odor association learning.