Molecular and cellular mechanisms of vomeronasal signaling in mammals

The mouse vomeronasal organ plays a critical role in chemosensory communication and regulates diverse social and sexual behaviors. However, many physiological mechanisms underlying vomeronasal chemosensory signaling remain elusive. Therefore, the overall aim of my thesis was to gain a deeper understanding of the basic mechanisms that control VNO physiology. Specifically, my research focused on HCN channel-mediated vomeronasal proton-sensing and its potential role in sensory gain control of social chemosignaling. Initially, I described that urinary pH depends on gender, sexual experience and estrus cycle stages. In particular, sexually experienced female mice display a generally more acidic urinary pH, which significantly decreases during estrus. These results indicate that urinary pH regulation in mice depends on parameters which also play an instructive role in conspecific chemosensory communication and vomeronasal signaling. In subsequent electrophysiological studies, I described a novel signaling mechanism in the mouse VNO by revealing that VSNs are proton-sensitive. Using whole-cell patch-clamp recordings in acute VNO tissue slices, I showed that acidic solutions dose-dependently activate most VSNs with threshold activation in the pH range of mouse urine. Surprisingly, my investigations suggest no substantial involvement of ‘classical’ proton-sensitive ion channels or TRPC2, the major vomeronasal signal transduction channel, in pH-dependent signaling. By characterizing different biophysical and pharmacological properties, I revealed that HCN channels play a critical role in mediating vomeronasal proton-sensitivity. I could show that acidic solutions dose-dependently shift HCN activation to more depolarized potentials leading to channel opening under resting conditions. Furthermore, I described that the biophysical properties such as kinetics, ionic conductance and IV relationship as well as the pharmacological profile of both vomeronasal Ih and proton-induced currents are similar. In addition, I showed that amplitudes of proton-induced responses are significantly correlated with proton-induced HCN activation thresholds. Together, this research revealed urinary pH regulations by endocrine parameters as well as HCN channel-mediated proton-dependent signaling in the mouse VNO. These findings indicate a critical role of vomeronasal proton-sensitivity in gain control of social chemosignaling. Thus, my thesis provides new insights into the underlying mechanisms controlling VNO physiology and chemosensory communication