Conditioned taste aversion (CTA) has been commonly used as a model of learning and memory. Traditionally, CTA approaches have used a paradigm that follows the model of classical conditioning. This involves presentation of a novel tastant, such as sucrose (conditioned stimulus, CS), followed by an intraperitoneal injection of lithium chloride that induces gastric malaise (unconditioned stimulus, US), which results in the aversion of sucrose (conditioned response, CR). However, a more natural classical conditioning paradigm involves the consumption, rather than injection, of the US by using a self-administration paradigm that allows for time-dependent analysis of formation, generalization, and extinction of CTA as it would occur naturally.
An appreciation of the anatomy of the taste pathway is critical in understanding CTA, as the learning is dependent on salient gustatory cues. Taste information begins with taste buds on the tongue and is sent to the brainstem via three cranial nerves: facial, glossopharyngeal, and the vagus. The first synapse of these cranial nerves is in the nucleus of the solitary tract, where gustatory and visceral information are processed separately. From here, taste information is sent to the parabrachial nucleus, where gustatory and visceral information have been shown to overlap. Therefore, the parabrachial nucleus is a key site of investigation concerning CTA, as it may be the first area where taste and gastrointestinal cues converge, leading to a learning event. Electrophysiology and immunohistochemistry techniques have been used to show changes in neuronal activity in taste nuclei in conditioned taste aversion, including the use of c-Fos as a method of labeling neurons that respond to a specified behavior.
The use of inbred strains of mice, specifically the common strains C57BL/6J (B6) and DBA/2J (D2), allows for the investigation of phenotypic variation and specific genes underlying the various components of CTA. B6 and D2 mice have previously been characterized in terms of various ingestive behaviors, making these mice ideal for this study. Learning-based differences between B6 and D2 mice have been seen in various tasks, including types of conditioning. Therefore, the following studies investigated the hypothesis that these two strains differ in various aspects of CTA, a form of learning and memory. First, we hypothesized that D2 mice will make a stronger association between the taste and malaise compared to B6 mice, and that such strain differences would be evident in both a behavioral and anatomical analysis. Second, we hypothesized that any strain differences seen in behavior will also be evident in c-Fos labeling following a CTA.
The following experiments tested the hypothesis that D2 mice would condition a stronger taste aversion than B6 mice, and that this strain difference would be evident in behavioral measures as well as in patterns of neuronal activation. We used a self-administration paradigm to condition a taste aversion to lithium chloride, and then tested the CTA the following day, where the CTA generalized to sodium chloride. More alterations in measures of licking behavior were seen in D2 mice as a result of a CTA, suggesting D2 mice conditioned a stronger aversion than B6 mice. Using c-Fos as a neuronal marker, we then compared patterns of activation in the parabrachial nucleus between the strains following various types of stimulation (visceral, gustatory, or combination). Results showed no strain differences except following the generalization test, where D2 showed overall more c-Fos than B6, and specifically showed more c-Fos in the external medial nucleus, which has been associated with aversive stimuli. These results suggest that NaCl, a previously palatable stimulus, had shifted to an aversive stimulus due to a CTA, but only in D2 mice.