Browsing by Author "Franks, Kevin M"

Understanding the roles that distinct neuron types play within a neural circuit will provide important mechanistic insight into understanding what the circuit does and how it does it. Piriform cortex (PCx) is the largest cortical recipient of odor information from the olfactory bulb (OB). It is thought to be the locus of odor perception and has been linked to several other crucial olfactory processing functions. However, the circuit mechanisms by which many of these ascribed functions occur as well as a comprehensive description of the role of PCx in olfaction remain unclear. PCx contains several different types of excitatory principal neurons. The two main types are semilunar cells (SL) and superficial pyramidal cells (PYR). SLs and PYRs have distinct morphologies, local connectivity, biophysical properties, and downstream projection targets, signifying potentially different roles in odor processing. An incisive study of if and how SLs and PYRs differentially encode odors will advance our understanding of PCx function and olfactory processing, as a whole.Odor processing in PCx has been hypothesized to occur in two sequential stages. First, SLs receive and integrate afferent OB inputs and then PYRs receive, transform, and transmit SL inputs to downstream regions. To probe if and how these two cell types differentially process odor information, I recorded from populations of optogenetically identified SLs and PYRs in awake, head-fixed mice. I then characterized their odor response properties, and selectively manipulated SL activity while mice were passively smelling odors and performing an odor-driven behavior. I found that SLs and PYRs received sensory information from the OB directly and simultaneously, and PYRs did not rely on SL activity to respond to odors, suggesting that these two cell types form parallel channels for processing odor information. Additionally, SLs and PYRs exhibited differences in odor response properties that were consistent with their distinct local connectivity, suggesting that distinct cell types differentially transform odor information. Finally, SLs and PYRs have distinct roles in mediating odor-driven behaviors. Together, my data show that SLs and PYRs form parallel channels for differentially processing odor information in and through PCx. More broadly, my findings provide evidence that PCx contains a functionally diverse population of neurons. Registering specific PCx functions to specific neuron types or subpopulations of neurons will provide a framework for determining what it is that PCx does for olfaction and how it does it.

Internal representations of the external world guide organisms in executing appropriate behavior. The olfactory system in mice is a prime model of this process as mice rely heavily on their sense of smell to survive and determine whether to approach and investigate or avoid and flee. Otherwise known as valence, this is the intrinsic quality of the attractiveness or averseness of an odor and is arguably the first aspect of description organisms use to describe and categorize odors. Even before this categorization, these odorants must be defined as salient. The salience of a stimulus is defined as the intensity of a stimulus and serves to direct the attention of an organism to focus their perceptual and cognitive resources on the pertinent sensory stimulus. Furthermore, many odors trigger instinctive aversive or attractive responses, thus carrying an innate, unlearned value.While several higher-order olfactory regions have been implicated in processing innate valence of odorants, none have been shown to harbor an internal representation of innate odor valence. The central amygdala (CeA) has largely been studied in the context of emotion and fear processing, with studies demonstrating encoding of both innate and learned valence to various stimuli. One subpopulation of cells in the CeA, activated by general anesthetics (CeAGA), has been shown to be responsive to innately salient pain stimuli. To probe whether cells within the CeA encode innate valence of odorant stimuli, I recorded odor-evoked neural responses from populations of optogenetically identified CeAGA cells in awake, head-fixed mice and characterized their odor response properties. I found that CeAGA neurons respond to a variety of odorants and encode innate odor salience. Because CeAGA receives substantial input from a region called the amygdala-piriform transition zone (AmPir), which has been implicated in predator odor processing, I recorded the odor-evoked neural responses from populations of cells within this region. I found that AmPir shows odor response properties similar to piriform and does not harbor a representation of innate odor salience. Finally, the strongly valent predator odor 2MT induces robust defensive behaviors in mice. To probe the role of CeA cells in these defensive behaviors, I optogenetically manipulated their activity in the presence of innately salient predator odor and pain stimuli. I found no robust changes in defensive behaviors to predator odor and pain stimuli. Together, these studies reveal the first evidence of innate odor categorization in the mouse through innate odor salience.