Activation of pre-synaptic M-type K+ channels inhibits [3H]d-aspartate release by reducing Ca2+ entry through P/Q-type voltage-gated Ca2+channels

In this study, the functional consequences of the pharmacological modulation of the M-current (I-KM) on cytoplasmic Ca2+ intracellular Ca(2+)concentration ([Ca2+](i)) changes and excitatory neurotransmitter release triggered by various stimuli from isolated rat cortical synaptosomes have been investigated. K(v)7.2 immunoreactivity was identified in pre-synaptic elements in cortical slices and isolated glutamatergic cortical synaptosomes. In cerebrocortical synaptosomes exposed to 20 mM [K+](e), the I-KM activator retigabine (RT, 10 mu M) inhibited [H-3]d-aspartate ([H-3]d-Asp) release and caused membrane hyperpolarization; both these effects were prevented by the I-KM blocker XE-991 (20 mu M). The I-KM activators RT (0.1-30 mu M), flupirtine (10 mu M) and BMS-204352 (10 mu M) inhibited 20 mM [K+](e)-induced synaptosomal [Ca2+](i) increases; XE-991 (20 mu M) abolished RT-induced inhibition of depolarization-triggered [Ca2+](i) transients. The P/Q-type voltage-sensitive Ca(2+)channel (VSCC) blocker omega-agatoxin IVA prevented RT-induced inhibition of depolarization-induced [Ca2+](i) increase and [H-3]d-Asp release, whereas the N-type blocker omega-conotoxin GVIA failed to do so. Finally, 10 mu M RT did not modify the increase of [Ca2+](i) and the resulting enhancement of [H-3]d-Asp release induced by [Ca2+](i) mobilization from intracellular stores, or by store-operated Ca(2+)channel activation. Collectively, the present data reveal that the pharmacological activation of I-KM regulates depolarization-induced [H-3]d-Asp release from cerebrocortical synaptosomes by selectively controlling the changes of [Ca2+](i) occurring through P/Q-type VSCCs.