protein that may be involved in the establishment and maintenance of specific neuronal connections in the brain. The expression of clustered protocadherins has been shown to be strongly linked with selection for ethanol preference in mice [31, 32]. In addition, the expression of PCDHB16 is increased in the mouse NAC (the same region we found ASE in this study) in response to cocaine [33]. TMEM25 encodes a transmembrane protein that is expressed in multiple brain regions, including the cerebellar cortex and hippocampus, as well as in neuroblastoma and brain tumors, and may be involved in the promotion of axon growth and the regulation of cell migration [34, 35]. SMPD4 encodes an enzyme sphingomyelinase that catalyzes the hydrolysis of sphingomyelin into phosphorylcholine and ceramide and is important in maintaining sphingolipid metabolism. It is one of the neutral sphingomyelinases whose activity is increased in HepG2 cells [36] and in astrocytes [37] in response to alcohol. Neutral sphingomyelinases are highly active in brain regions [38] and are suggested to play an important role in neurological disorders [39, 40]. For example, in brain injury, the activity of neutral sphingomyelinase is induced and the ceramide level accumulates in astrocytes after cerebral ischemia [39]. Another study
