Non-specific effects of the MEK inhibitors PD098,059 and U0126 on glutamate release from hippocampal synaptosomes

In order to investigate a role for the extracellular-signal-regulated kinases 1 and 2 (ERK1/2) on hippocampal neurotransmitter release, we studied the effect of commonly used MEK (mitogen-activated protein kinase [MAPK]/ERK kinase) inhibitors, PD098,059 and U0126, on depolarization-induced glutamate release. PD098,059 inhibited glutamate release front hippocampal synaptosomes stimulated with 15 mM KCl in a concentration-dependent manner. At the same range of concentrations, PD098,059 inhibited basal and KCl-stimulated ERK1/2 phosphorylation. U0126, however, did not significantly affect KCl-evoked glutamate release at concentrations shown to inhibit ERK activity. Nonetheless. U0126 unspecifically potentiated depolarization-induced Ca(2+)-independent glutamate release, which masked a small dose-dependent inhibitory effect on the Ca(2+)-dependent release. PD098,059 reduced the [Ca(2+)](i) response to KCl by partially inhibiting Ca(2+) entry through N- and P-/Q-type voltage-gated Ca(2+) channels, whereas U0126 did not affect depolarization-induced Ca(2+) influx. To overcome the unspecific effect of PD098,059 on Ca(2+) entry, we studied the effect of both MEK inhibitors on glutamate release stimulated by a Ca(2+) ionophore. PD098,029 and U0126 showed a small dose-dependent inhibitory effect on ionomycin-induced glutamate release, at concentrations shown to inhibit ionomycin-stimulated ERK phosphorylation. These findings uncover new unspecific actions for both MEK inhibitors and suggest a minor role for ERK in modulating glutamate release in the hippocampus. (C) 2002 Elsevier Science Ltd. All rights reserved.