In SH-SY5Y neuroblastoma cell bodies and growth cones the actin dynamics of cholinergically induced lamellar protrusion is demonstrated by live-imaging. Failure of this reaction in cells over-expressing a dominant negative RAC1-mutant (RAC1(T17N)) confirmed that the actin-dynamics of lamellar protrusion is also, in these cells, RAC1-dependent. Pretreatment of untransfected cells with 20 microM UO126 for 2 h, shown to down-regulate the basic level and to completely inhibit cholinergic activation of mitoge-activated kinase (MAPK; erk1/erk2), had no effect neither on spontaneous nor on induced lamellar protrusions. When p-JNK, whose basic activity is very low and not enhanced by a cholinergic stimulation of the cells for up to 30 min, is activated in the presence of 50 microM anisomycin, both spontaneous and cholinergically induced lamellar protrusive activity is attenuated. These data indicate that the RAC1-controlled cascades promoting lamellar protrusion are independent of MAPK activity and partially down-regulated by p-JNK.