Faculty Mentor: Matt Wachowiak (Neurobiology and Anatomy, University of Utah)

Olfactory sensory neurons (OSNs) are the primary sensory neurons responsible for detecting odor molecules expressing a vast array of odorant receptors (ORs), each tuned to respond to specific chemical structures enabling the perception of diverse odors¹. In nature, odors are mixtures of molecules, leading to potentially complex interactions at the receptor, such as antagonism, which may influence downstream odor encoding. Odorant-driven antagonism occurs when an odorant receptor cannot bind with a ligand (agonist) because a similar molecule (antagonist) blocks the receptor binding pocket². Previous work utilizing binary mixture in vitro assays in heterologous expression systems found robust evidence for antagonism in a class I receptor, which are tuned to carboxylic acids³.

However, in vitro assays are removed from the biological system, so it remains unclear whether antagonism manifests in vivo. Therefore, the aim of this project was to test for evidence of odorant- driven antagonism in vivo at the level of OSNs. If antagonism occurs in vivo, we expect to see a decrease in the response of OSNs to an agonist when presented with an antagonist. We used two- photon imaging in awake, head-fixed mice expressing genetically encoded calcium reporters in OSNs to record responses to agonists, antagonists, and binary agonist + antagonist mixtures. Agonist and antagonist pairs were chosen based on previous in vitro antagonism experiments. During experimental trials, agonist and binary mixture (agonist + antagonist) presentations alternated to account for potential habituation, and mixtures were presented using a priming protocol with the antagonist presented by itself prior to the mixture. Our preliminary data yielded suggestive evidence that antagonism occurred in vivo with nonanoic acid and 2-bromohexanoic acid acting as antagonists for valeric acid and tiglic acid-responding OSNs, respectively. The findings of this study are preliminary and do not significantly conclude that antagonism occurs in vivo, so additional work is needed to understand how robust antagonism is in vivo. Identifying antagonism in vivo is the first step to disentangling how complex interactions at the receptor level influence odor mixture encoding.

Footnotes

1. Oka, Y., Omura, M., Kataoka, H. & Touhara, K. Olfactory receptor antagonism between odorants. EMBO J 23, 120–126 (2004).

2. Oka, Y., Nakamura, A., Watanabe, H. & Touhara, K. An odorant derivative as an antagonist for an olfactory receptor. Chem Senses 29, 815–822 (2004).

3. Fukutani, Y. et al. Antagonistic interactions between odorants alter human odor perception. Current Biology 33, 2235-2245.e4 (2023).