Title: Representations and Transformations of Odor Information in the Mouse Olfactory System
Description: For a wide variety of organisms on the planet, the sense of smell is of critical importance for survival. The mouse olfactory system mediates both learned and innate odor-driven behaviors, including activities as diverse as the localization of food sources, the avoidance of predators, and the selection of mates. How a chemical stimulus in the environment ultimately leads to the generation of an appropriate behavioral response, however, remains poorly understood. All of these behaviors begin with the binding of an odorant in the external environment to receptors on sensory neurons in the olfactory epithelium. These sensory neurons transmit this odor information to neurons in the olfactory bulb via spatially stereotyped axonal projections, and a subset of these bulbar neurons, mitral and tufted cells, in turn transmit this information to a number of higher brain regions implicated in both learned and innate odor-driven behaviors, including the piriform cortex and amygdala. Previous work has revealed that odorants drive activity in unique, sparse ensembles of neurons distributed across the piriform cortex without apparent spatial preference.

The patterns of neural activity observed, however, do not reveal whether mitral and tufted cell projections from a given glomerulus to piriform are segregated or distributed, or whether they are random or determined. Distinguishing between these possibilities is important for understanding the function of piriform cortex: a random representation of odor identity in the piriform could accommodate learned olfactory behaviors, but cannot specify innate odor-driven responses. In addition, behavioral studies in which the function of the amygdala has been compromised have found that innate odor-driven behaviors are disrupted by these manipulations while learned odor-driven behaviors are left intact, strongly suggesting a role for the amygdala in innate olfactory responses. How odor information is represented in the amygdala, as well as the amygdala's exact role in the generation of olfactory responses, however, remain poorly understood. We therefore developed a strategy to trace the projections from identified glomeruli in the olfactory bulb to these higher olfactory centers.

Electroporation of TMR dextran into single glomeruli has permitted us to define the neural circuits that convey olfactory information from specific glomeruli in the olfactory bulb to the piriform cortex and amygdala. We find that mitral and tufted cells from every glomerulus elaborate similar axonal arbors in the piriform. These projections densely fan out across the cortical surface in a homogeneous manner, and quantitative analyses fail to identify features that distinguish the projection patterns from different glomeruli. In contrast, the cortical amygdala receives spatially stereotyped projections from individual glomeruli. The stereotyped projections from each glomerulus target a subregion of the posterolateral cortical nucleus, but may overlap extensively with projections from other glomeruli. The apparently random pattern of projections to the piriform and the determined pattern of projections to the amygdala are likely to provide the anatomic substrates for distinct odor-driven behaviors mediated by these two brain regions.

The dispersed mitral and tufted cell projections to the piriform provide the basis for the generation of previously observed patterns of neural activity and suggest a role for the piriform cortex in learned olfactory behaviors, while the pattern of mitral and tufted cell projections to the posterolateral amygdala implicate this structure in the generation of innate odor-driven behaviors. We have also developed high-throughput methods for imaging odor-evoked activity in targeted populations of neurons in multiple areas of the olfactory system to investigate how odor information is represented and transformed by the mouse brain. We have used a modified rabies virus that drives expression of GCaMP3, a calcium-sensitive indicator of neural activity, to image odor-evoked responses from mitral and tufted cells, as well as a modified adenoassociated virus that drives expression of GCaMP3 to image odor-evoked responses from neurons in piriform cortex. These imaging methods have permitted us to examine odor-evoked responses in a transgenic mouse where 95% of sensory neurons express a single kind of olfactory receptor (M71).

In these mice, there is a 1,000-fold increase in sensory neurons expressing the M71 receptor ligand acetophenone, and a 20-fold reduction in neurons expressing olfactory receptors from the endogenous repertoire. These M71 transgenic mice provide a useful tool for examining the role that the normally stereotyped pattern of sensory neuron input to the bulb plays in olfactory processing, as well as how odor information is transformed as is moves from the sensory periphery to the cortex. In control mice, odors evoke activity in unique ensembles of spatially distributed, narrowly tuned mitral and tufted cells, and the number of cells responding to odor increases linearly with stimulus concentration. Surprisingly, despite the fact that there is a significant decrease in sensory neuron activity in response to odors other than acetophenone in M71 transgenics, a wide variety of odorants are able to evoke mitral and tufted cell activity in these mice. Furthermore, the number of cells responding to these odors as well as the magnitude of these odor-evoked responses are higher in M71 transgenics compared to controls.

However, despite a massive increase in acetophenone-evoked sensory neuron input to the bulb in M71 transgenics, mitral and tufted cell responses to acetophenone are similar in M71 transgenics and controls. Our results provide evidence for excitatory mechanisms that amplify weak sensory neuron input as well as inhibitory mechanisms that suppress strong, pervasive odor-evoked input, suggesting that a major role of the olfactory bulb is to aid in the comprehensive detection and refinement of olfactory signals from the environment. Despite the fact that the representation of odor in the olfactory bulb of M71 transgenic mice differs from that observed in controls, we find the representations of odor in the piriform cortex of M71 transgenic mice and controls is quantitatively indistinguishable. Our results suggest that circuits intrinsic to the piriform significantly transform the representation of odor information as it moves from the olfactory bulb to the piriform cortex. Moreover, in comparison to the olfactory bulb, the piriform encodes odor in a more sparse, distributed manner within a much narrower dynamic range.

The nature of the representation of odor we observe in piriform cortex further supports a role for this area in mediating odor discrimination and associative odor-driven behaviors. The work described in this thesis has provided insight into the way odor is represented in several areas of the mouse olfactory system, clues about how odor information is transformed as it passes through the brain, and the role that different areas of the olfactory system play in odor-driven perception and behavior. In the future, the novel techniques and methods described in this thesis can be applied to the study of many different areas of the mammalian brain, giving our work the potential to have a significant impact on our understanding of how patterns of neural activity may ultimately underlie the generation of perceptions, emotions, and behaviors.