Type of Document Dissertation Author Samuelsen, Chad L. Author's Email Address email@example.com URN etd-09212009-161414 Title Chemosensory Processing In The Amygdala Degree Doctor of Philosophy Department Biological Science, Department of Advisory Committee
Advisor Name Title Michael Meredith Committee Chair Elaine Hull Committee Member Laura Keller Committee Member Zuoxin Wang Committee Member Colleen Kelley Outside Committee Member Keywords
- Biologically Relevant
- Social Behavior
Date of Defense 2009-09-14 Availability unrestricted AbstractRodents depend on the main olfactory and accessory olfactory systems to detect and process chemical-communication signals important for complex reproductive and social behaviors. As the medial amygdala is the first site of convergence of these two pathways in the brain, examining its role in chemosensory processing is of primary importance. Previous experiments have shown that the hamster medial amygdala exhibits a categorical response to different types of chemical signals. The use of immediate early gene (IEG) expression to assess neural activity in different brain regions showed that chemical signals from the same species (conspecific) activated anterior and posterior medial amygdala, while chemical signals from other species (heterospecific) increased activity in only the anterior medial amygdala. The experiments discussed below aim to provide information about how the mouse medial amygdala responds to categories of chemical signals, how important the main and accessory olfactory systems are to the medial amygdala response and whether oxytocin (which has been shown to be important in a variety of social behaviors) is necessary for the medial amygdala categorization of chemical signals.
Upon exposure of male mice to chemical-communication signals, I found that conspecific chemosignals (male, female mouse urine) increased immediate early gene-protein (IEG) expression in both anterior and posterior medial amygdala of male mice, whereas most heterospecific chemosignals (e.g.: hamster vaginal fluid, steer urine) increased expression only in anterior medial amygdala. This categorization of responses in medial amygdala conforms to the previously reported findings in male hamsters. The same characteristic pattern of IEG expression appeared in the medial amygdala of each species in response to conspecific stimuli for that species. These results suggested that the amygdala categorizes stimuli according to the biological relevance for the tested species. Thus, a heterospecific predator (cat collar) stimulus, which elicited behavioral avoidance in mice, increased IEG expression in mouse posterior medial amygdala (like conspecific stimuli). Further analysis suggests reproduction related and potentially threatening stimuli produce increased IEG expression in different sub-regions of posterior medial amygdala (dorsal and ventral, respectively). These patterns of IEG expression in medial amygdala may provide glimpses of a higher-level processing of chemosensory signals beyond the primary-level selectivity of chemosensory neurons, the secondary sorting in main and accessory olfactory bulbs and the tertiary sorting by the medial amygdala into “biologically relevant and non-relevant” categories.
Both non-volatile and volatile chemical-communication signals may be detected by the vomeronasal organ, which sends projections to the accessory olfactory bulb and on to the medial amygdala. Results of the first experiment (above) argue that the mouse medial amygdala sorts complex chemosensory information categorically, according to its biological relevance (salience). In order to determine the role of AOS in categorization, male mice underwent vomeronasal removal surgery (VNX) or a sham-operation (SHAM) and then were exposed to conspecific (male and female mouse urine) or heterospecific (hamster vaginal fluid and worn cat collar) chemical stimuli. As with the mice in the above experiment, SHAM mice exhibited different IEG expression patterns in the medial amygdala dependent upon the biological relevance of the chemical stimuli. However, regardless of biological relevance, vomeronasal organ removal eliminated the different IEG response patterns in the medial amygdala to any of the chemical stimuli. Interestingly, VNX also disrupted the avoidance of (an unfamiliar) predator odor, worn cat collar. These experiments show that the medial amygdala response to these tested chemical signals is dependent upon an intact vomeronasal organ.
Normal function of the neuropeptide oxytocin (OT) in the medial amygdala is necessary for social recognition. In the final set of experiments, male mice having undergone intracerebroventricular cannulation (i.c.v.) were injected with either PBS (control) or oxytocin antagonist (OTA) and exposed to conspecific (female mouse urine) and heterospecific (steer urine and worn cat collar) chemical stimuli. As in the above experiments, PBS-injected mice exhibited different IEG expression patterns in the medial amygdala dependent upon the biological relevance of the chemical stimuli. However, OTA injection eliminated the increase in IEG expression in the medial amygdala to all of the tested conspecific or heterospecific stimuli. Importantly, OTA injection disrupted defensive and non-defensive behaviors after exposure to the unfamiliar predator odor, worn cat collar. The disruption of the social/individual recognition behavior in male mice deficient in OT may be due to the inability of the medial amygdala to process all biologically relevant chemical-communication signals.
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