Fast Adaptation in Mouse Olfactory Sensory Neurons Does Not Require the Activity of Phosphodiesterase

Vertebrate olfactory sensory neurons rapidly adapt to repetitive odorant stimuli. Previous studies have shown that the principal molecular mechanisms for odorant adaptation take place after the odorant-induced production of cAMP, and that one important mechanism is the negative feedback modulation by Ca²⁺-calmodulin (Ca²⁺-CaM) of the cyclic nucleotide-gated (CNG) channel. However, the physiological role of the Ca²⁺-dependent activity of phosphodiesterase (PDE) in adaptation has not been investigated yet. We used the whole-cell voltage-clamp technique to record currents in mouse olfactory sensory neurons elicited by photorelease of 8-Br-cAMP, an analogue of cAMP commonly used as a hydrolysis-resistant compound and known to be a potent agonist of the olfactory CNG channel. We measured currents in response to repetitive photoreleases of cAMP or of 8-Br-cAMP and we observed similar adaptation in response to the second stimulus. Control experiments were conducted in the presence of the PDE inhibitor IBMX, confirming that an increase in PDE activity was not involved in the response decrease. Since the total current activated by 8-Br-cAMP, as well as that physiologically induced by odorants, is composed not only of current carried by Na⁺ and Ca²⁺ through CNG channels, but also by a Ca²⁺-activated Cl⁻ current, we performed control experiments in which the reversal potential of Cl⁻ was set, by ion substitution, at the same value of the holding potential, −50 mV. Adaptation was measured also in these conditions of diminished Ca²⁺-activated Cl⁻ current. Furthermore, by producing repetitive increases of ciliary's Ca²⁺ with flash photolysis of caged Ca²⁺, we showed that Ca²⁺-activated Cl⁻ channels do not adapt and that there is no Cl⁻ depletion in the cilia. All together, these results indicate that the activity of ciliary PDE is not required for fast adaptation to repetitive stimuli in mouse olfactory sensory neurons.