Behavioral responses to taste stimuli include those leading to ingestion (mouth movements, tongue protrusion, etc.) and rejection (gapes, etc.). The neural circuitry required for these behaviors is contained within the brainstem. Several forebrain regions are connected to brainstem taste centers, but the functional roles of these connections have not been determined. Therefore, the specific goal of this study is to determine the behavioral roles of the descending projections from forebrain structures to gustatory centers in the brainstem. The general experimental approach will be to activate the descending projections with implanted electrodes in conscious rats and to observe changes in taste-related behaviors (particularly to salty and bitter stimuli). To understand the mechanisms of the behavioral effects, changes in the location, number and type of neurons in the gustatory brainstem, activated by forebrain stimulation, will be determined using a combination of anatomical and molecular techniques. It is expected that the results of this study will improve our understanding of the neural mechanisms underlying taste-related behaviors and provide a foundation for future research on the role of these pathways in more complex behaviors. The broader impacts of the proposed activities include the involvement of a diverse undergraduate student population in scientific research, enhanced undergraduate student preparation for graduate work in the Biological Sciences, and published findings to disseminate the results of the experiments. Beyond those impacts, the proposed activities will increase interaction among scientists and students at Stetson University, enhance learning, teaching and training in classrooms and laboratories, and promote outreach to local middle- and high-school teachers and their students.
The overall goal of this project was to improve the understanding of the neural mechanisms controlling behavioral responses to taste input. Taste is a critical part of the evaluation of food that has entered the oral cavity. Behavioral responses to taste input can be categorized as those that indicate that the food is palatable and will be ingested and those that indicate that the food is unpalatable and will be rejected. In their simplest form, these behaviors are referred to as ingestive and aversive taste reactivity (TR) behaviors. It is known that TR behaviors are controlled by neurons in the brainstem that receive taste input and control motor output to the muscles responsible for the behaviors. It also is known that several forebrain structures are interconnected with these gustatory brainstem nuclei and modulate electrical activity of neurons in these areas to taste input. These forebrain areas include the central amygdala (CeA), lateral hypothalamus (LH), and gustatory cortex (GC). However, the effects of projections from these forebrain areas to the brainstem on TR behaviors are unknown. Therefore, the scientific goals of this project were to determine the effects of the CeA, LH, and GC on TR behaviors and to identify neurons in the brainstem that are influenced by descending projections from these forebrain areas. The experimental approach involved implanting electrodes into one of the targeted forebrain areas and cannulas into the mouth, so that a brain region could be stimulated in a conscious rat while a solution was being infused into its mouth. The TR behaviors were videotaped and analyzed to determine if forebrain stimulation altered the responses to water as well as salty, sweet, sour, bitter, and umami tastes. The neurons in the brainstem activated by the electrical stimulation and intra-oral infusions were identified using immunohistochemistry to the Fos protein that is expressed in a subset of activated neurons. The data reveal that stimulation of the CeA, LH, and GC in conscious rats elicits TR behaviors without intra-oral infusion of a solution. CeA and LH stimulation caused predominantly ingestive behaviors whereas GC stimulation caused both ingestive and aversive behaviors. In addition, it was found that CeA and LH stimulation alters some TR responses to taste solutions. Specifically, the most profound effects were that CeA stimulation increased the aversive TR responses to salty and sour tastes and LH stimulation reduced aversive TR responses to bitter. These results are the first demonstration that the pathways descending from the CeA, LH, and GC can alter TR behaviors, and they suggest that these pathways have different roles in modulating the behavioral responses to taste input. Some of the behavioral effects of intra-oral infusion of taste solutions and brain stimulation were accompanied by changes in the number of Fos-immunoreactive neurons in the brainstem providing a starting point for the identification of the neural substrate underlying these behavioral changes. This project also included an evaluation of the location of neurons in the GC activated by intra-oral infusion of taste solutions as well as those that control TR behaviors in conscious rats. The mapping of GC neurons responsive to different tastes could identify populations of neurons responsible for perceiving different taste qualities. Overall, more neurons in the anterior GC were activated by intra-oral infusion compared to the posterior GC suggesting an uneven distribution of taste-responsive neurons. Electrical stimulation of the anterior GC elicited TR behaviors while posterior stimulation did not, also supporting a role for the anterior GC in behavioral responses to taste input. Future studies will investigate the role of different regions of the GC in processing information about different tastes and controlling TR behaviors. One of the broader impacts of this project was to provide an opportunity for undergraduate students to participate in scientific research. Thirteen undergraduate Biology majors at Stetson University, two undergraduates at a local community college (Daytona State College), and a high school student participated in this project. In addition, this project supported outreach and science literacy activities at a museum on campus including the purchase of a computer for interactive displays about neuroscience and participation of the PI and students in a science café series for local adults interested in science and Brain Awareness week activities for local children and teachers.
