Arcadlin (Acad/Protocadherin-8) is a non-clustered d2-protocadherin of the cadherin superfamily. Acad is induced quickly by neural activity and is known as a molecule that reduces spine density in the hippocampal neurons in vivo and in vitro. Pyramidal neurons in the layer II of the piriform cortex strongly expresses Acad mRNA. The piriform cortex is often kindled electrically for seizure generations. We hypothesize that the neural activity of the piriform neurons induces Arcadlin, which in turn modulates their spine morphology. In order to investigate this possibility, we examined the dendritic morphology of the pyramidal neurons in the piriform cortex in Acad^−/− mice. Contrary to our expectation, Acad^−/− mice showed a lower spine density than WT mice in the pyramidal cells of piriform cortex. The change in spine density was most obviously observed in thin spines, and in the dendritic zone distal to the cell body. The data suggest that Acad does not suppress the piriform pyramidal cells at least in unkindled status. We will further examine whether the induced Acad suppresses the piriform pyramidal cells under kindled condition.