CHARACTERISTICS AND POSTNATAL-DEVELOPMENT OF A HYPERPOLARIZATION-ACTIVATED INWARD CURRENT IN RAT HYPOGLOSSAL MOTONEURONS IN-VITRO

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 h ypoglossal motoneurons (HMs). 2. In young adult HMs (>P21), a noninact ivating, time- and voltage-dependent inward current was evident during hyperpolarizing voltage steps to membrane potentials negative to simi lar to -65 mV from depolarized-holding potentials [V-h = -56.2 +/- 1.0 (SE) mV]. The averaged reversal potential (E(rev)) of the inward curr ent, estimated using an extrapolation procedure, was -38.8 +/- 2.9 mV (n = 5), suggesting that a mixed cationic current underlies inward rec tification in HMs. 3. The voltage dependence of I-h activation was det ermined from tail current relaxations that followed a family of voltag e steps to different membrane potentials. Normalized tail current ampl itudes were well-fitted with a single Boltzman function with a half-ac tivation 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-depen dent. Activation proceeded more quickly with larger hyperpolarizing vo ltage 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 wa s 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 ex tracellular barium. The current amplitude near half-activation (simila r 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 h alf-activation, I-h was similar to 10-fold larger in adult (greater th an or equal to P21; n = 20) than in neonatal HMs (less than or equal t o P8; n = 7). Input conductance (G(N)) was only threefold higher in th e same sample of HMs. There were no apparent differences in the voltag e dependence or E(rev) of I-h between neonatal and older HMs. These re sults suggest that the increased amplitude of I-h results from an incr ease in I-h current density. The postnatal changes in I-h expression m ay account, in part, for differences in some intrinsic properties of n eonatal and adult HMs (e.g., lower membrane resistivity and higher G(N ), larger depolarizing ''sag'', shorter duration action potentials and afterhyperpolarizations in adult HMs).