Chad L. Samuelsen, Ph.D. - US grants

Chemosensory signals activate brain regions along the vomeronasal and main olfactory pathways. In male hamsters, socially relevant chemosignals.from the same species of animal (conspecific) activate the anterior medal amygdala (MeA) and the posterior medial amygdala (MeP) and do not activate the GABATergic intercalated nucleus (ICN). Socially non-relevant chemosignals from other species activate MeA and ICN, but fail to activate MeP. Oxytocin knockout mice (OTKO) fail to recognize a conspecific that they have encountered previously. The cFos patterns in the medial amygdala suggest a heterospecific-like response to the stimulus animal. Therefore it is possible that oxytocin (OT) mediates the signal from the vomeronasal system in the medial amygdala allowing for "downstream" continuation of the signal. These and other data suggest that oxytocin (OT) may affect medial amygdala responses to socially-relevant and socially nonrelevant stimuli differently. This proposal will investigate OT effects in medial amygdala in response to conspecific and heterospecific chemosensory stimulation.

DESCRIPTION (provided by applicant): Integration of sensory information from multiple modalities is believed to enhance the detection of convergent stimuli. This multimodal integration was originally theorized to occur only in higher order cortical areas, such as prefrontal cortex. However, recent research has shown multimodal sensory responses in every primary sensory cortex, including gustatory cortex (GC). GC is the primary cortex responsible for processing information related to taste and is known to code multisensory information related to taste, such as temperature, touch and anticipatory cues. However, the most conspicuous sensory interaction, that of taste and olfaction, has not been explored at the GC circuit-level. Taste and olfaction are intimately linked. Anatomical and neuroimaging studies have begun to describe the synergistic integration of these two modalities and suggest GC as a primary integrative chemosensory node. Despite this great progress, very little information is available on whether and how single neurons in GC integrate multimodal chemosensory integration. The proposed research is a three step approach, all of which rely upon multielectrode techniques to record ensembles of single neurons in GC while rats intraorally sample tastes and odors. The first experiment is designed to determine how taste and odor are represented by single neurons in GC. After a better understanding of GC chemosensory response, the second experiment will test the hypothesis that odor-taste experience modulates chemosensory activity in GC. The third experiment continues along this trajectory to explore how incongruent odor-taste mixtures suppress chemosensory activity in GC. Altogether, the experiments outlined in this R03 proposal will lay a foundation for an in depth understanding of the circuit-level integration of multimodal chemosensory information in GC of alert rodents. If successful, this research will elucidate how single neurons in GC process ecologically synergistic chemosensory information and will establish GC as a primary area of multisensory integration of stimuli related to food.

PROJECT SUMMARY ABSTRACT Although interactions between the senses of smell and taste are a key factor for guiding food choices, the neural mechanisms underlying the multisensory integration of odors and tastes remain largely unknown. A more thorough understanding of these neurobiological processes will provide better insight into diseases characterized by unhealthy food choices, such as obesity or diabetes. Eating food simultaneously activates the olfactory and gustatory systems to generate enduring odor-taste associations. The primary cortical area for taste, gustatory cortex (GC), is a principal site of convergent gustatory and olfactory information. Recent work from our lab determined that functionally distinct populations of neurons in GC represent different properties of individual unpaired odors or tastes. However, how neurons in GC represent odor-taste mixtures, as well as the neural mechanisms that underlie integration and processing of smell and taste remain unclear. Recent behavioral and physiological studies show that cortico-cortical interactions between sensory cortices are fundamental to multisensory integration, suggesting that projections from the functionally distinct anterior (aPC) and posterior (pPC) regions of piriform cortex (i.e., olfactory cortex) modulate multimodal chemosensory processing in GC. Using rats as a model system, this proposal will combine behaving electrophysiology and optogenetic techniques to investigate how the circuit between neurons in GC and the functionally distinct regions of piriform cortex mediates multisensory integration of odors and tastes. The Specific Aims will test the following hypotheses: Aim 1: Different populations of neurons in GC encode the chemical identity and hedonic value of odor-taste mixtures. Aim 2: Neurons in GC that respond to both odors and tastes receive aPC input representing the chemical identity of odor-taste mixtures. Aim 3: Neurons in GC that respond to both odors and tastes receive pPC input representing the hedonic value of odors and odor-taste mixtures. The results of each Specific Aim will contribute fundamental knowledge about the cortico-cortical circuitry underlying the integration of smell and taste and provide a necessary prelude for investigating the role that these cortico-cortical circuits play in guiding food choices.