Because U0126 inhibits the MEK5-catalyzed activation of ERK5, as well as the MEK1- and MEK2-catalyzed activation of ERK1 and ERK2 (Bain et al., 2007; Mody et al., 2001), we also used the more specific, structurally unrelated MEK1 and MEK2 inhibitor PD184352 at concentrations in which it inhibits MEK1 and MEK2 but not MEK5 (Bain et al., 2007; Mody et al., 2001).