Role of calcium conductances on spike afterpotentials in rat trigeminal motoneurons

Intracellular recordings were obtained from rat trigeminal motoneurons in slice preparations to investigate the role of calcium conductances in the depolarizing and hyperpolarizing spike afterpotential (ADP and mAHP, respectively). The mAHP was suppressed by bath application of 1 μM apamin, 2 mM Mn²⁺, and 2 mM Co²⁺, and also by intracellular injection of ethylene glycol-bis(b-aminoethylenether)-N,N,N',N'-tetraacetic acid (EGTA), suggesting that the potassium conductance generating the mAHP is activated by Ca²⁺ influx. Mn²⁺ (2 mM) or Cd²⁺ (500 μM) reduced the ADP, whereas the ADP amplitude was increased by raising extracellular Ca²⁺ concentration from 2 to 8 mM by bath application of Ba²⁺ (0.5-5 mM) and by intracellular injection of EGTA. This would suggest that Ca²⁺ itself is likely to be the charge carrier generating the ADP. Focal application of ω-conotoxin GVIA (10-30 μM) suppressed the mAHP and enhanced the ADP, whereas focal application of ω-agatoxin IVA (10-100 μM) reduced the ADP amplitude without apparent effects on the mAHP. We conclude that Ca²⁺ influx through ω- agatoxin IVA-sensitive calcium channels is at least in part responsible for the generation of the ADP and that Ca²⁺ influx through ω-conotoxin GVIA- sensitive calcium channels contributes to the generation of the mAHP. Because of the selective suppression of the ADP and mAHP by ω-agatoxin IVA and ω- conotoxin GVIA, respectively, it is suggested that both calcium channels are separated geometrically in rat trigeminal motoneurons.