Functional Profiling of Olfactory Sensory Neurons: Electrophysiological Characterization of Human Olfactory Epithelium and Maturation-Dependent Changes in Mouse Neuronal Excitability

The olfactory system is crucial for the detection of chemical cues from the environment, allowing species survival. Olfactory sensory neurons (OSNs) within the olfactory epithelium (OE) are responsible for odorant detection and signal transmission to the brain. To investigate olfaction, rodents, along with other species such as amphibians and fishes, have been extensively used as laboratory models. In the first part of this thesis, we provided the first electrophysiological characterization of human OSNs. Current knowledge about human OE is primarily confined to its morphology and molecular profile. However, little is known about the functional properties of human OSNs and supporting cells. We obtained acute slices of human OE from nasal biopsies and demonstrated their viability for whole-cell patch-clamp recordings. We measured voltage-gated currents from both OSNs and supporting cells in voltage-clamp configuration. Current-clamp protocols allowed us to assess the excitability of OSNs, which exhibited diverse firing patterns. Moreover, we demonstrated that these acute slices are also feasible for studying olfactory transduction, as we obtained the first electrophysiological responses of human OSNs upon odorant stimulation. Stimulation with a phosphodiesterase inhibitor elicited neuronal inward currents and action potentials, providing evidence that cyclic adenosine monophosphate (cAMP) is involved in the transduction pathway of human olfaction. In the second part of the thesis, we investigated immature OSNs from the mouse OE. The OE has the capability to continuously regenerate throughout life. To better characterize epithelial regeneration, a deeper knowledge of immature OSNs is required. While gene remodelling and morphological rearrangements are well established, changes in electrophysiological properties across maturation, remain largely unexplored. Using an electrophysiological approach, we explored the intrinsic properties of immature OSNs. Through loose-patch recordings, we demonstrated that immature OSNs are already endowed with a spontaneous activity. Currentclamp experiments showed that these neurons are excitable, although displaying lower excitability and slower action potential kinetics compared to mature OSNs. Both electrophysiological and transcriptomic analyses revealed differences in voltage-gated currents along development. Focusing on voltage-gated Na+ and K+ channels, we found the emergence of tetrodotoxin-resistant Na+ currents and transient A-type K+ currents when neurons become mature, likely influencing changes in firing behaviour. Altogether, these findings provide a comprehensive electrophysiological characterization of human OSNs, contributing to a deeper understanding of olfactory mechanisms in humans, and expand the current knowledge of OSN functional maturation through the functional description of immature OSNs in a mouse model.