Properties and function of low- and high-voltage-activated Ca2+ channels in hypoglossal motoneurons.

Calcium influx through voltage-gated Ca2+ channels plays an important role in neuronal function. In a thin-slice preparation of neonatal rat hypoglossal motoneurons (HMs) we recorded Ba2+ currents through voltage-gated Ca2+ channels using the whole-cell configuration of the patch-clamp technique. We found that HMs have low-voltage-activated (LVA) and at least three types of high-voltage-activated (HVA) Ca2+ channels (omega-Aga-IVA sensitive, omega CgTx sensitive, and dihydropyridine sensitive), based on pharmacological and voltage-dependent properties. Of the Ca2+ current activated at 0 mV from a holding potential of -70 mV, approximately one-half was omega-Aga-IVA (200 nM) sensitive, one-third was omega-CgTx (3 microM) sensitive, whereas only 6% was DHP (nimodipine; 10 microM) sensitive. The residual current, after applying these three antagonists, had characteristics of LVA Ca2+ currents. Based on this pharmacology we found that Ca2+ entry during a single action potential (AP) through LVA Ca2+ channels has a different role from CA2+ entry through HVA Ca2+ channels. Ca2+ influx through omega-Aga-IVA-sensitive and omega-CgTx-sensitive HVA Ca2+ channels activates Ca(2+)-activated K+ channels responsible for the AP afterhyperpolarization. On the other hand, Ca2+ entry through LVA Ca2+ channels is responsible for spike afterdepolarization and provides Ca2+ for the Ca(2+)-activated K+ channels that contribute to AP repolarization.