K_V1/D-type potassium channels inhibit the excitability of bronchopulmonary vagal afferent nerves

Abstract: The K_V1/D-type potassium current (I_D) is an important determinant of neuronal excitability. This study explored whether and how I_D channels regulate the activation of bronchopulmonary vagal afferent nerves. The single-neuron RT-PCR assay revealed that nearly all mouse bronchopulmonary nodose neurons expressed the transcripts of α-dendrotoxin (α-DTX)-sensitive, I_D channel-forming K_V1.1, K_V1.2 and/or K_V1.6 α-subunits, with the expression of K_V1.6 being most prevalent. Patch-clamp recordings showed that I_D, defined as the α-DTX-sensitive K⁺ current, activated at voltages slightly more negative than the resting membrane potential in lung-specific nodose neurons and displayed little inactivation at subthreshold voltages. Inhibition of I_D channels by α-DTX depolarized the lung-specific nodose neurons and caused an increase in input resistance, decrease in rheobase, as well as increase in action potential number and firing frequency in response to suprathreshold current steps. Application of α-DTX to the lungs via trachea in the mouse ex vivo vagally innervated trachea–lungs preparation led to action potential discharges in nearly half of bronchopulmonary nodose afferent nerve fibres, including nodose C-fibres, as detected by the two-photon microscopic Ca²⁺ imaging technique and extracellular electrophysiological recordings. In conclusion, I_D channels act as a critical brake on the activation of bronchopulmonary vagal afferent nerves by stabilizing the membrane potential, counterbalancing the subthreshold depolarization and promoting the adaptation of action potential firings. Down-regulation of I_D channels, as occurs in various inflammatory diseases, may contribute to the enhanced C-fibre activity in airway diseases that are associated with excessive coughing, dyspnoea, and reflex bronchospasm and secretions. Key points: The α-dendrotoxin (α-DTX)-sensitive D-type K⁺ current (I_D) is an important determinant of neuronal excitability. Nearly all bronchopulmonary nodose afferent neurons in the mouse express I_D and the transcripts of α-DTX-sensitive, I_D channel-forming K_V1.1, K_V1.2 and/or K_V1.6 α-subunits. Inhibition of I_D channels by α-DTX depolarizes the bronchopulmonary nodose neurons, reduces the minimal depolarizing current needed to evoke an action potential (AP) and increases AP number and AP firing frequency in response to suprathreshold stimulations. Application of α-DTX to the lungs ex vivo elicits AP discharges in about half of bronchopulmonary nodose C-fibre terminals. Our novel finding that I_D channels act as a critical brake on the activation of bronchopulmonary vagal afferent nerves suggests that their down-regulation, as occurs in various inflammatory diseases, may contribute to the enhanced C-fibre activity in airway inflammation associated with excessive respiratory symptoms.