4-Aminopyridine-sensitive outward currents in preBotzinger complex neurons influence respiratory rhythm generation in neonatal mice

We measured a low-threshold, inactivating K+ current, i.e. A-current (I-A), in respiratory neurons of the preBotzinger complex (preBotC) in rhythmically active slice preparations from neonatal C5713L/6 mice. The majority of inspiratory neurons (21/34 = 61.8%), but not expiratory neurons (1/8 = 12.5%), expressed I-A. In whole-cell and somatic outside-out patches I-A activated at -60 mV (half-activation voltage measured - 16.3 mV) and only fully inactivated above -40 MV (half-inactivation voltage measured - 85.6 mV), indicating that I-A can influence membrane trajectory at baseline voltages during respiratory rhythm generation in vitro. 4-Aminopyridine (4-AP, 2 mm) attenuated I-A in both whole-cell and somatic outside-out patches. In the context of rhythmic network activity, 4-AP caused irregular respiratory-related motor output on XII nerves and disrupted rhythmogenesis as detected with whole-cell and field recordings in the preBotC. Whole-cell current-clamp recordings showed that 4-AP changed the envelope of depolarization underlying inspiratory bursts (i.e. inspiratory drive potentials) from an incrementing pattern to a decrementing pattern during rhythm generation and abolished current pulse-induced delayed excitation. These data suggest that I-A opposes excitatory synaptic depolarizations at baseline voltages of approximately -60 mV and influences the inspiratory burst pattern. We propose that I-A promotes orderly recruitment of constituent rhythmogenic neurons by minimizing the activity of these neurons until they receive massive coincident synaptic input, which reduces the periodic fluctuations of inspiratory activity.