Modulation of pheromone responses in vomeronasal sensory neurons

Chemosensation allows individuals to perceive chemical stimuli from the surrounding environment leading the survival of all the species. The vomeronasal organ (VNO) is a peripheral sensory organ present in many mammals that is involved in the detection of pheromones. Pheromones are substances released by animals which trigger behavior and physiological changes of other individuals of the same species. The binding of pheromones is transduced into electrical signals by specialized neurons that transfer to the central nervous system the information carried by social cues. Ligand binding to vomeronasal receptors on microvilli of the dendritic knobs of vomeronasal sensory neurons (VSNs) triggers a biochemical cascade that activates a PLC-dependent second messenger transduction that ultimately results in the TRPC2 channel gating allowing the inward flow of sodium and calcium. The increase in cytosolic calcium concentration plays several roles in signal transduction, involving the activation of other ion channels. Previous studies showed that calcium activated chloride currents are activated by cytosolic calcium increase in mouse VSNs, and that both TMEM16A and TMEM16B, two proteins forming calcium-activated chloride channels, are co-expressed in microvilli of VSNs, but their physiological role is still uncertain. With the help of two mouse models, TMEM16A cKO and TMEM16B KO mice, we provide evidence of the role of both channels in spontaneous firing and evoked activity in VSNs. Extracellular recordings in loose-patch configuration showed that firing pattern of spontaneous activity was affected in VSNs from TMEM16A cKO mice, showing less activity with respect to WT neurons, while VSNs from TMEM16B KO mice showed spontaneous firing comparable to WT. During urine-evoked activity both channels are involved in modulation of firing. Inter spike interval (ISI) distribution of evoked activity showed that VSNs from TMEM16A cKO and TMEM16B KO fire with shorter intervals than WT neurons. We conclude that calcium-activated chloride current modulates the firing of VSNs varying the effect with an activity-dependent mode. Adaptation is an additional mechanism of response modulation common in sensory systems. To evaluate the molecular players involved in sensory adaptation in VNO during early pheromone processing, we used whole-cell recordings in the current-clamp mode to measure the action potential firing of VSNs stimulated by diluted urine. We performed a paired pulse protocol with different inter-pulse intervals (IPIs), ranging from 5 to 60 seconds. We found that VSNs reduced their firing rate depending on the duration of preceding stimulation and on IPIs. Surprisingly, similar results were obtained also upon stimulation with direct injection of current, which does not activate neither the receptors nor the signaling cascade, suggesting that the adaptation would be, at least partially, independent from the initial sensory transduction cascade. We reveal that sodium currents in VSNs undergo to a slow recovery from inactivation that last several seconds mirroring the time scale of paired pulse experiments. Our results provide a foundation for future work investigating the precise molecular mechanisms of modulation of pheromones signaling at peripheral level during early stages of firing in VSNs.