Developmental changes in calcium/calmodulin-dependent inactivation of calcium currents at the rat calyx of Held

Abstract

Ca²⁺-binding to calmodulin (CaM) causes facilitation and/or inactivation of recombinant Ca²⁺ channels. At the rat calyx of Held, before hearing onset, presynaptic Ca²⁺ currents (I_pCa) undergo Ca²⁺/CaM-dependent inactivation during repetitive activation at around 1 Hz, implying that this may be a major cause of short-term synaptic depression. However, it remains open whether the Ca²⁺/CaM-dependent inactivation of I_pCa persists in more mature animals. To address this question, we tested the effect of CaM inhibitors on the activity-dependent modulation of I_pCa in calyces, before (postnatal day (P) 7–9) and after (P13–15) hearing onset. Our results indicate that the CaM-dependent I_pCa inactivation during low-frequency stimulation, and the ensuing synaptic depression, occur only at calyces in the prehearing period. However, CaM immunoreactivity in P8 and P14 calyces was equally strong. Even at P13–15, high frequency stimulation (200–500 Hz) could induce I_pCa inactivation, which was attenuated by EGTA (10 mm) or a CaM inhibitor peptide loaded into the terminal. Furthermore, the CaM inhibitor peptide attenuated a transient facilitation of I_pCa preceding inactivation observed at 500 Hz stimulation, whereas it had no effect on sustained I_pCa facilitations during trains of 50–200 Hz stimulation. These results suggest that the Ca²⁺/CaM-dependent I_pCa modulation requires a high intraterminal Ca²⁺ concentration, which can be attained at immature calyces during low frequency stimulation, but only during unusually high frequency stimulation at calyceal terminals in the posthearing period.