Persistent Sodium Current, Membrane Properties and Bursting Behavior of Pre-Bötzinger Complex Inspiratory Neurons In Vitro

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Abstract
We measured persistent Na+current and membrane properties of bursting-pacemaker and nonbursting inspiratory neurons of the neonatal rat pre-Bötzinger complex (pre-BötC) in brain stem slice preparations with a rhythmically active respiratory network in vitro. In whole-cell recordings, slow voltage ramps (≤100 mV/s) inactivated the fast, spike-generating Na+ current and yielded N-shaped current-voltage relationships with nonmonotonic, negative-slope regions between −60 and −35 mV when the voltage-sensitive component was isolated. The underlying current was a TTX-sensitive persistent Na+ current ( I NaP) since the inward current was present at slow voltage ramp speeds (3.3–100 mV/s) and the current was blocked by 1 μM TTX. We measured the biophysical properties of I NaP after subtracting the voltage-insensitive “leak” current ( I Leak) in the presence of Cd2+ and in some cases tetraethylammonium (TEA). Peak I NaP ranged from −50 to −200 pA at a membrane potential of −30 mV. Decreasing the speed of the voltage ramp caused time-dependent I NaPinactivation, but this current was present at ramp speeds as low as 3.3 mV/s. I NaP activated at −60 mV and obtained half-maximal activation near −40 mV. The subthreshold voltage dependence and slow inactivation kinetics of I NaP, which closely resemble those of I NaP mathematically modeled as a burst-generation mechanism in pacemaker neurons of the pre-BötC, suggest that I NaP predominantly influences bursting dynamics of pre-BötC inspiratory pacemaker neurons in vitro. We also found that the ratio of persistent Na+conductance to leak conductance ( g NaP/ g Leak) can distinguish the phenotypic subpopulations of bursting pacemaker and nonbursting inspiratory neurons: pacemaker neurons showed g NaP/ g Leak> g NaP/ g Leakin nonpacemaker cells ( P < 0.0002). We conclude that I NaP is ubiquitously expressed by pre-BötC inspiratory neurons and that bursting pacemaker behavior within the heterogeneous population of inspiratory neurons is achieved with specific ratios of these two conductances, g NaP and g Leak.