The Role of the Hyperpolarization-Activated Current in Modulating Rhythmic Activity in the Isolated Respiratory Network of Mice

Abstract

We examined the role of the hyperpolarization-activated current (I_h) in the generation of the respiratory rhythm using a spontaneously active brainstem slice of mice. This preparation contains the hypoglossus (XII) nucleus, which is activated in-phase with inspiration and the pre-Bötzinger complex (PBC), the presumed site for respiratory rhythm generation. Voltage-clamp recordings (n= 90) indicate that cesium (Cs) (5 mm) blocked 77.2% of theI_hcurrent, and ZD 7288 (100 μm) blocked 85.8% of theI_hcurrent. This blockade increased the respiratory frequency by 161% in Cs and by 150% in ZD 7288 and increased the amplitude of integrated population activity in the XII by 97% in Cs and by 162% in ZD 7288, but not in the PBC (Cs, by 19%; ZD 7288, by −4.56%). All inspiratory PBC neurons (n= 44) recorded in current clamp within the active network revealed a significantly decreased frequency of action potentials during the interburst interval and an earlier onset of inspiratory bursts afterI_hcurrent blockade. However, hyperpolarizing current pulses evoked only in a small proportion of inspiratory neurons (0% of type I; 29% of type II neurons) a depolarizing sag. Most of the neurons expressing anI_hcurrent (86%) were pacemaker neurons, which continued to generate rhythmic bursts after inactivating the respiratory network pharmacologically with CNQX alone or with CNQX, AP-5, strychnine, bicuculline, and carbenoxolone. Cs and ZD 7288 increased the frequency of pacemaker bursts and decreased the frequency of action potentials between pacemaker bursts. Our findings suggest that theI_hcurrent plays an important role in modulating respiratory frequency, which is presumably mediated by pacemaker neurons.