The 5-HT 6 receptor (5-HT 6 R) is one of the most recently cloned serotonin receptors, and it plays important roles in Alzheimer disease, depression, and learning and memory disorders. However, unlike the other serotonin receptors, the cellular mechanisms of 5-HT 6 R are poorly elucidated relative to its significance in human brain diseases. Here, using a yeast two-hybrid assay, we found that the human 5-HT 6 R interacts with Jun activation domain-binding protein-1 (Jab1). We also confirmed a physical interaction between 5-HT 6 R and Jab1 using glutathione S-transferase pulldown, fluorescence resonance energy transfer, co-immunoprecipitation, and immunocyto(histo)-chemistry assays. The manipulation of Jab1 expression using Jab1 small interference RNA decreased 5-HT 6 R-mediated activity and cell membrane expression of 5-HT 6 R, whereas overexpression of Jab1 produced no significant effect. In addition, we demonstrated that the activation of 5-HT 6 R induced the translocation of Jab1 into the nucleus and increased c-Jun phosphorylation and the interaction between Jab1 and c-Jun. Furthermore, we found that 5-HT 6 R and Jab1 were up-regulated in middle cerebral artery occlusion-induced focal cerebral ischemic rats and in cultured cells exposed to hypoxic insults, suggesting possible protective roles for 5-HT 6 R and Jab1. These findings suggest that Jab1 provides a novel signal transduction pathway for 5-HT 6 R and may play an important role in 5-HT 6 Rmediated behavior changes in the brain.Neurotransmitters and neurohormones regulate diverse and myriad brain functions ranging from rapid modulation of ligand-gated ion channels to long term modulation such as gene expression, behavior, and mood changes. Serotonin (5-HT) 2 is known to be one of the key neurotransmitters related to mood changes. The serotonergic system has also been implicated in the neurobiological control of learning and memory. Therefore, it emerges as a key player in affective, cognitive, complex sensory, and motor functions (1). 5-HT mediates its diverse physiological responses through seven distinct receptor families: the 5-HT 1 , 5-HT 2 , 5-HT 3 , 5-HT 4 , 5-HT 5 , 5-HT 6 , and 5-HT 7 receptors. With the exception of the 5-HT 3 receptor, all of these receptors are members of the G-protein-coupled receptor (GPCR) superfamily (2). Among the 5-HT receptors, the 5-HT 6 receptor (5-HT 6 R) is coupled to a stimulatory G␣ (G␣ S ) protein. This receptor increases cAMP formation and then activates cAMP-dependent protein kinase (3). 5-HT 6 R is abundantly distributed in the brain, especially in the limbic region (4), and it has a high affinity for antipsychotic compounds and tricyclic antidepressants (5). These preliminary reports imply that 5-HT 6 R has a significant role in the control of mood and emotion in the central nervous system. To date, most findings suggest that 5-HT 6 R plays crucial roles in neurological disorders, including Alzheimer disease, depression, and learning and memory disorders (6 -9). Selective antagonists of 5-HT 6 R improved me...