1. Single-electrode voltage clamp recordings in a rat brain stem slice preparation were used to determine the characteristics and postnatal development of a hyperpolarization-activated inward current (I-h) in hypoglossal motoneurons (HMs). 2. In young adult HMs (>P21), a noninactivating, time- and voltage-dependent inward current was evident during hyperpolarizing voltage steps to membrane potentials negative to similar to -65 mV from depolarized-holding potentials [V-h = -56.2 +/- 1.0 (SE) mV]. The averaged reversal potential (E(rev)) of the inward current, estimated using an extrapolation procedure, was -38.8 +/- 2.9 mV (n = 5), suggesting that a mixed cationic current underlies inward rectification in HMs. 3. The voltage dependence of I-h activation was determined from tail current relaxations that followed a family of voltage steps to different membrane potentials. Normalized tail current amplitudes were well-fitted with a single Boltzman function with a half-activation at -79.8 +/- 0.7 mV and slope factor = 5.3 +/- 0.3 (n = 8). 4. Time constants of I-h activation and deactivation were voltage-dependent. Activation proceeded more quickly with larger hyperpolarizing voltage steps; time constants averaged 389, 181, and 134 ms at -69, -82, and -95 mV, respectively (n = 6). I-h deactivated during depolarizing voltage steps from hyperpolarized holding potentials. Deactivation was faster with larger depolarizing steps; time constants averaged 321, 215, and 107 ms at -80, -71, and -62 mV, respectively (n = 4). 5. I-h was sensitive to extracellular cesium but relatively insensitive to extracellular barium. The current amplitude near half-activation (similar to -84 mV) was almost completely blocked (to 11% of control) by Cs+ (3 mM, n = 3) but was reduced to only 85 and 60% in 0.5 (n = 2) and 2 mM Ba2(+) (n = 3), respectively. 6. There was a marked increase in the amplitude of I-h during postnatal development of HMs. Measured near half-activation, I-h was similar to 10-fold larger in adult (greater than or equal to P21; n = 20) than in neonatal HMs (less than or equal to P8; n = 7). Input conductance (G(N)) was only threefold higher in the same sample of HMs. There were no apparent differences in the voltage dependence or E(rev) of I-h between neonatal and older HMs. These results suggest that the increased amplitude of I-h results from an increase in I-h current density. The postnatal changes in I-h expression may account, in part, for differences in some intrinsic properties of neonatal and adult HMs (e.g., lower membrane resistivity and higher G(N), larger depolarizing ''sag'', shorter duration action potentials and afterhyperpolarizations in adult HMs).
