The role of the hyperpolarization-activated current in modulating rhythmicactivity in the isolated respiratory network of mice

We examined the role of the hyperpolarization-activated current (I-h) in th e generation of the respiratory rhythm using a spontaneously active brainst em slice of mice. This preparation contains the hypoglossus (XII) nucleus, which is activated in-phase with inspiration and the pre-Botzinger complex (PBC), the presumed site for respiratory rhythm generation. Voltage-clamp r ecordings (n = 90) indicate that cesium (Cs) (5 mM) blocked 77.2% of the I- h current, and ZD 7288 (100 mu M) blocked 85.8% of the Ih current. This blo ckade increased the respiratory frequency by 161% in Cs and by 150% in ZD 7 288 and increased the amplitude of integrated population activity in the XI I by 97% in Cs and by 162% in ZD 7288, but not in the PBC (Cs, by 19%; ZD 7 288, by -4.56%). All inspiratory PBC neurons (n = 44) recorded in current c lamp within the active network revealed a significantly decreased frequency of action potentials during the interburst interval and an earlier onset o f inspiratory bursts after I-h current blockade. However, hyperpolarizing c urrent 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 an I-h current (86%) were pacemaker neurons, which continued to generate rhythmic bursts after inactivating the respiratory network pharmac ologically 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 the I-h current plays an important role in modula ting respiratory frequency, which is presumably mediated by pacemaker neuro ns.