Activation of Presynaptic M-Type K(+) Channels inhibits [H]D-Aspartate release by Reducing Ca(2+) entry through P/Q-type voltage-gated Ca(2+) channels

In this study, the functional consequences of the pharmacological modulation of the M-current (IKM) on cytoplasmic Ca2+ intracellular Ca2+concentration ([Ca2+]i) changes and excitatory neurotransmitter release triggered by various stimuli from isolated rat cortical synaptosomes have been investigated. Kv7.2 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 IKM activator retigabine (RT, 10 lM) inhibited [3H]D-aspartate ([3H]D-Asp) release and caused membrane hyperpolarization; both these effects were prevented by the IKM blocker XE-991 (20 lM). The IKM activators RT (0.1– 30 lM), flupirtine (10 lM) and BMS-204352 (10 lM) inhibited 20 mM [K+]e-induced synaptosomal [Ca2+]i increases; XE-991 (20 lM) abolished RT-induced inhibition of depolarizationtriggered [Ca2+]i transients. The P/Q-type voltage-sensitive Ca2+channel (VSCC) blocker x-agatoxin IVA prevented RTinduced inhibition of depolarization-induced [Ca2+]i increase and [3H]D-Asp release, whereas the N-type blocker x-conotoxin GVIA failed to do so. Finally, 10 lM RT did not modify the increase of [Ca2+]i and the resulting enhancement of [3H]D-Asp release induced by [Ca2+]i mobilization from intracellular stores, or by store-operated Ca2+channel activation. Collectively, the present data reveal that the pharmacological activation of IKM regulates depolarization-induced [3H]D-Asp release from cerebrocortical synaptosomes by selectively controlling the changes of [Ca2+]i occurring through P/Q-type VSCCs.