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 inves tigated using intracellular recording and patch-clamp techniques in a brain stem slice preparation of neonatal rats (0- 1 5 days old). 2. Th e action potential was followed by an afterdepolarization (ADP). The A DP was voltage dependent, increasing with membrane hyperpolarization. Raising the extracellular Ca2+ concentration or replacing Ca2+ with Ba 2+ increased the ADP amplitude, whereas replacement of Ca2+ with Mn2blocked it. The ADP was partially reduced by amiloride and low concent rations of Ni2+. 3. The firing behavior of individual neonatal HMs was influenced by membrane potential. From depolarized potentials, HMs fi red tonically in response to a depolarizing current pulse, whereas fro m 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 disap peared by the tenth postnatal day. In addition, prominent rebound depo larizations characterized the response of neonatal motoneurons to hype rpolarizing prepulses. 4. Pharmacological characterization of the rebo und depolarization demonstrated that it was calcium dependent. Its amp litude was insensitive to tetrodotoxin and it was eliminated by replac ement 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 t etraethylammonium. The high threshold calcium spikes were greatly enha nced 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+ cu rrents. Low voltage-activated (LVA) and a high voltage-activated (HVA) calcium current components were identified on the basis of their volt age thresholds for activation, kinetics of inactivation, and pharmacol ogical sensitivity. 7. The LVA calcium current began to activate at ar ound -60 mV and inactivated nearly completely within 100 ms. Complete steady-state inactivation occurred at potentials more positive than -6 0 mV. The LVA current was selectively reduced by 1 mM amiloride (3 1 % ). 8. A larger-amplitude calcium current activated at potentials aroun d -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 muM omega-conotoxin GVIA, whereas a smaller fraction was blocked by 10 muM nifedipine. 9. It is concluded that neonatal HMs have disti nct calcium currents that participate in their electrical activity. Th e properties of the LVA current are consistent with its role in reboun d depolarization and burst-firing behavior. Activation of the HVA calc ium current during the action potential gives rise to calcium influx r esponsible for the calcium-sensitive potassium current that underlies the afterhyperpolarization (see accompanying paper). Both the LVA and the HVA calcium currents contribute to the ADP.