Figure 4. Time course of extracellular signal-regulated kinase (ERK) activation by D-Ala2, N-Me-Phe4, Gly5-ol enkephalin (DAMGO). Mu receptor-expressing C6 cells were treated with the indicated concentrations of DAMGO for the times noted. PMA (4-phorbol 12-myristate 13-acetate) and control exposures lasted 30 min.