In the present study, we tested the hypothesis that the activation of imidazoline I(1)-receptor, which is coupled to phosphatidylcholine-specific phospholipase C, results in downstream activation of mitogen-activated protein kinase (p42(mapk) and p44(mapk) isoforms) in PC12 cells. PC12 cells pretreated with nerve growth factor (50 ng/ml, 48 h) to initiate neuronal differentiation were incubated with [methyl-3H]choline and [3H]myristate. Activation of imidazoline I(1) receptor by rilmenidine (10 microM) caused time-dependent increases in diacylglycerol accumulation and phosphocholine release. The Western blotting analysis showed that rilmenidine (10 microM) produced a time-dependent activation of p42(mapk) and p44(mapk) that reached its maximum at 15 min and returned to control levels after 30 min. This finding was confirmed by immunofluorescence labeling of activated mitogen-activated protein kinase in the same model system. Efaroxan (imidazoline I(1)-receptor antagonist) or tricyclodecan-9-yl-xanthogenate (D609, phosphatidylcholine-specific phospholipase C inhibitor) attenuated the phosphorylation of p42(mapk) and p44(mapk) induced by rilmenidine. Nerve growth factor-induced phosphorylation of both mitogen-activated protein kinase isoforms was not affected by D609. These results support the hypothesis that the activation of the imidazoline I(1) receptor coupled phosphatidylcholine-specific phospholipase C results in the downstream activation of mitogen-activated protein kinase.