ERK1/2 activation triggered by melatonin, ICOA‐9 and ICOA‐13. Detection of phospho‐ERK1/2 (P‐ERK1/2) in HEK293 cells expressing hMT1 (A, B), hMT2 (C), mMT1 (D) or mMT2 (E) receptors incubated for 0 to 60 min with melatonin (100 nM), ICOA‐9 (1 μM), ICOA‐13 (1 μM) (Western blot; A) or at 5 min at different concentrations of the indicated ligands (Alpha‐Screen assay; B, C, D, E). Data are expressed as mean ± SEM from three to four independent experiments [(B) melatonin n = 4, ICOA‐9 and ICOA‐13 n = 3; (C) all groups n = 3; (D, E) melatonin n = 4, ICOA‐9 n = 3, ICOA‐13 n = 4]. Receptor expression levels were 155 ± 62, 52 ± 22, 136 ± 63 and 112 ± 22 fmol·mg−1 of protein for hMT1, hMT2, mMT1 and mMT2 receptors respectively.