Expression of p-ERK in the (A) dura mater, (B) TG and (C) TNC following NTG infusion. As shown in the histogram, the phosphorylation of ERK following NTG infusion of the dura mater, TG and TNC demonstrated a temporal profile. Significantly higher levels of p-ERK were found in the dura mater, TG and TNC 30 min after NTG administration compared with the controls. The p-ERK levels gradually decreased and were close to the basal level by 3 h. (**P<0.01, compared with the NS group; n=6 in each group; error bars indicate standard deviation). p-ERK, phosphorylated extracellular signal-regulated kinase; TNC, trigeminal nucleus caudalis; TG, trigeminal ganglion; NTG, nitroglycerin; NS, vehicle-treated rats.