ABSTRACT. Nicotinamide attenuates neuronal cell death related to focal cerebral ischemic injury. This study investigated whether nicotinamide exerts a neuroprotective effect through the activation of Raf-mitogen-activated protein kinase kinase (MEK)-ERK and its downstream targets, including p90 ribosomal S6 kinase (p90RSK) and Bad. Adult male Sprague-Dawley rats were treated with nicotinamide (500 mg/kg) or vehicle 2 hr after the onset of middle cerebral artery occlusion (MCAO). Brains were collected 24 hr after MCAO. In the present study, nicotinamide significantly reduces the volume of infarct regions and decreases the number of positive cells by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining in the cerebral cortex. Nicotinamide prevents injury-induced decrease in Raf-1, MEK1/2, and ERK1/2 phosphorylation. As part of the downstream cascade, nicotinamide inhibits the injury-induced decrease in p90RSK and Bad phosphorylation. Moreover, nicotinamide prevents the injury-induced increase in cleaved caspase-3 levels. These findings suggest that nicotinamide protects neuronal cells against cerebral ischemic injury and that MEK-ERK-p90RSK cascade activation by nicotinamide contributes to these neuroprotective effects. Ischemic brain injury leads to serious cellular damage through the imbalance of energy metabolism and the generation of oxidative stress 1, 4, 15. Nicotinamide, vitamin B3 amide form, is the precursor for NAD+ and is the necessary factor for cellular function and energy metabolism 27. Nicotinamide reduces the generation of reactive oxygen species (ROS) and attenuates cell damage from cerebral ischemic injury [2,13,20,23]. Nicotinamide prevents necrosis and apoptosis by inhibiting DNA destruction [28]. Moreover, nicotinamide exerts a cytoprotective effect in neurodegenerative diseases such as Alzheimer's disease and traumatic brain injury [3,6,12].Mitogen-activated protein (MAP) kinases (MAPKs, Erk1/2) are members of the serine and threonine protein kinase family involved in many cellular programs including cell differentiation, proliferation, survival, and cell death [22]. In the presence of growth factor or mitogen, MAPKs are phosphorylated and activated by MAPK-kinase (MAPKK, Raf-1), which in turn are phosphorylated and activated by MAPKK-kinase (MAPKKK, MEK1/2). The up-stream enzymes of MAPK are Raf and MEK. While, down-stream targets of MAPK are 90 kDa ribosomal S6 kinase (p90RSK) and transcription factor 18,22]. ERK1/2 leads to the phosphorylation of p90RSK, which then leads to the phosphorylation of the pro-apoptotic bcl-2 family, Bad [5,11,24]. The Raf-MEK-ERK signal cascade attenuates apoptotic cell death through the phosphorylation of the downstream targets, p90RSK and Bad. Previous studies reported that nicotinamide exerts a neuroprotective effect by activation of cellular survival signaling pathway including protein kinase B/Akt [7,8]. Although previous studies have demonstrated the neuroprotective effect of nicotinamide, little data is available regarding th...