Neural circuitry underlying expression of Fos-like immunoreactivity in intermediate nucleus of the solitary tract following expression of taste aversion learning

Taste aversion learning results from the association between a novel, palatable taste and transient malaise such that re-exposure to the taste results in the animal displaying rejection behaviors. Expression of a conditioned taste aversion (CTA) is also associated with the expression of fos -like immunoreactivity (FLI) in the medial region of the intermediate nucleus of the solitary tract (iNTS) following subsequent exposure to a taste that has been previously paired with LiCl. This cellular marker of activation has been used to identify brain structures involved in CTA. Recent findings indicate that different conditioning methods may involve different neural circuits. Amygdala is necessary for both behavioral expression of CTA and FLI when an intraoral infusion (I/O) of the tastant is used at the time of conditioning. However, amygdala is not necessary for the behavioral expression of CTA when bottle conditioning is used. Therefore, the first aim of this dissertation was to determine if expression of FLI following CTA was dependent upon conditioning method. The second and third aims characterize the efferent and afferent projections of CTA-induced FLI cells to obtain a better understanding of the neural circuitry underlying taste aversion learning.; The results of the first study showed that I/O and bottle methods generated aversions that were comparable, when judged by the behavioral response of solution rejection. However, elevations in FLI were seen only in animals conditioned with the I/O method. Furthermore, expression of FLI was localized to the parvicellular subdivision of iNTS.; The second and third studies used several techniques to characterize these projections including anterograde and retrograde tract tracing, double labeling for retrograde tract tracer and FLI, and lesions. FLI cells in iNTS receive direct projections from amygdala, paraventricular hypothalamus and lateral hypothalamus. The targets of nearly 80% of FLI cells were accounted for, of these, over half of these FLI cells project to structures in the pons and forebrain while the remaining cells project locally within the medulla. Finally, these cells are not necessary for the oromotor rejection responses associated with expression of taste aversion learning.