<p>The specificity of anti-G-protein-coupled receptor (GPCR) antibodies has been tested in knockout animals; many of these antibodies are nonspecific (Pradidarcheep et al. 2009). Due to the lack of specific antibodies against the GPCR proteins, tissue distribution and subcellular localization of endogenous GPCR proteins in their native state are largely unknown. Anti-neuropeptide Y Y₅ receptor antibody, which did not react with Y₅ knockout brain tissue, revealed Y₅-immunoreactivities as dot-like structures adjacent to the plasma membrane in the wild-type brain tissues (Murase et al. 2017). Here, we examined the expression pattern of endogenous neuropeptide Y Y₁ receptor using an anti-Y₁ antibody, the specificity of which was validated by a Y₁ knockout mouse. The anti-Y₁ antibody reacted with the wild-type brain sections and detected Y₁-immunoreactivities. Y₁/Y₅ double-positive neurons were observed in most brain regions examined here. In these neurons, Y₁ immunoreactivities appeared as dot-like structures, as did Y₅. The anti-Y₁ antibody recognized the intracellular part of endogenous Y₁ molecules rather than the cell membrane-bound form, indicating the difficulties and demand for establishing antibodies against the physiologically active form of Y₁ molecules in vivo.</p>