A pyrimidone derivative represented by formula (I) or a salt thereof, or a solvate thereof or a hydrate thereof: wherein: Y represents two hydrogen atoms, a sulphur atom, an oxygen atom or a C1-2 alkyl group and a hydrogen atom; Z represents a bond, an oxygen atom, a nitrogen atom substituted by a hydrogen atom or a C1-3 alkyl group, a sulphur atom, a methylene group optionally substituted by one or two groups chosen from a C1-6 alkyl group, a hydroxyl group, a C1-6 alkoxy group, a C1-2 perhalogenated alkyl group or an amino group; R1 represents a 2, 3 or 4-pyridine ring or a 2, 4 or 5-pyrimidine ring, the ring being optionally substituted by a C1-6 alkyl group, a C1-6 alkoxy group or a halogen atom; R2 represents a hydrogen atom, a C1-6 alkyl group or a halogen atom; R3 represents a benzene ring or a naphthalene ring; the rings being optionally substituted R4 represents a hydrogen atom, a C1-6 alkyl group, a C3-7 cycloalkyl group, a C3-7 cycloalkyl-C1-6 alkyl group, a COO (C1-6-alkyl) group, these group being optionally substituted; R5 represents a hydrogen atom or a C1-6 alkyl group and n represents 0 to 3 and their therapeutic use for the treatment of neurodegenerative diseases caused by abnormal activity of GSK3β.