Calcium conductances and their role in the firing behavior of neonatal rat hypoglossal motoneurons.

1. The role of calcium conductances in action potential generation and repetitive firing behavior of hypoglossal motoneurons (HMs) was investigated using intracellular recording and patch-clamp techniques in a brain stem slice preparation of neonatal rats (0-15 days old). 2. The action potential was followed by an afterdepolarization (ADP). The ADP was voltage dependent, increasing with membrane hyperpolarization. Raising the extracellular Ca2+ concentration or replacing Ca2+ with Ba2+ increased the ADP amplitude, whereas replacement of Ca2+ with Mn2+ blocked it. The ADP was partially reduced by amiloride and low concentrations of Ni2+. 3. The firing behavior of individual neonatal HMs was influenced by membrane potential. From depolarized potentials, HMs fired tonically in response to a depolarizing current pulse, whereas from more hyperpolarized membrane potentials (more negative than -70 mV), a subset of HMs fired an initial burst of action potentials followed by a prolonged afterhyperpolarization and tonic firing. The incidence of burst-firing behavior was highest among young motoneurons and disappeared by the tenth postnatal day. In addition, prominent rebound depolarizations characterized the response of neonatal motoneurons to hyperpolarizing prepulses. 4. Pharmacological characterization of the rebound depolarization demonstrated that it was calcium dependent. Its amplitude was insensitive to tetrodotoxin and it was eliminated by replacement of Ca2+ with Mn2+ or addition of Ni2+. Amiloride (1-1.5 mM) had no effect on the rebound response or burst firing. 5. The presence of high-threshold calcium spikes was detected at all postnatal ages, but only after blockade of outward currents with intra- or extracellular tetraethylammonium. The high-threshold calcium spikes were greatly enhanced when Ba2+ replaced Ca2+. 6. Calcium currents of neonatal HMs were characterized in whole-cell patch-clamp recordings of thin medullary slices under conditions that minimized voltage-dependent Na+ and K+ currents. Low voltage-activated (LVA) and a high voltage-activated (HVA) calcium current components were identified on the basis of their voltage thresholds for activation, kinetics of inactivation, and pharmacological sensitivity. 7. The LVA calcium current began to activate at around -60 mV and inactivated nearly completely within 100 ms. Complete steady-state inactivation occurred at potentials more positive than -60 mV. The LVA current was selectively reduced by 1 mM amiloride (31%). 8. A larger-amplitude calcium current activated at potentials around -35 mV. Inactivation of this HVA current was slower than that of the LVA current and incomplete. About 1/3 of this current was sensitive to 1 microM omega-conotoxin GVIA, whereas a smaller fraction was blocked by 10 microM nifedipine.(ABSTRACT TRUNCATED AT 400 WORDS)