Calcium-activated chloride channels in mouse vomeronasal sensory neurons

In vomeronasal sensory neurons, signal transduction occurs in microvilli that are present at the neuron’s apical surface. The binding of pheromone to vomeronasal receptors causes an increase of the intracellular calcium concentration by calcium entry through TRPC2. An important issue is the impact of Ca2+ entry in pheromonal transduction. In the first part of this thesis we have investigated the functional role played by the increase in intracellular Ca2+ concentration in the apical region of vomeronasal sensory neurons. By taking advantage of flash photolysis of caged calcium restricted to the apical region of neurons, we have measured a calcium-activated current with the whole-cell voltage-clamp technique. Our results demonstrated that a large current is indeed activated by calcium in the apical region of mouse vomeronasal sensory neurons and our immunohistochemistry data has revealed the presence of the proteins TMEM16A and TMEM16B, responsible for calcium-activated chloride channels, in the microvilli of vomeronasal sensory neurons. Therefore we have concluded that calcium-activated chloride channels are present at high density in the region where signal transduction occurs and therefore may play an important role in vomeronasal transduction. In the second part of this thesis we have characterized in more detail the calcium activated currents in mouse vomeronasal sensory neurons using the whole-cell voltage-clamp technique in the presence of various intracellular Ca2+ concentrations. From the dose-response relation we determined that the Ca2+ concentration necessary to activate 50% of the maximal current was 1.4 µM at -100 mV and 0.6 µM at +100 mV. From ion selectivity experiments, we found that the current is carried by anions. Moreover, we demonstrated that some of the commonly used Cl- channel blockers, NFA and CaCCinh-A01, do inhibit the Ca2+-activated current in vomeronasal sensory neurons. Further studies with knockout mice for TMEM16A or TMEM16B will be necessary to establish the physiological role of these channels in vomeronasal transduction.