Differentiation and distribution of acetylcholinesterase molecular forms in the mouse cochlea.

The activities of the globular and asymmetric forms of acetylcholinesterase (AChE) were measured in the whole cochlea and cochlear turns of the developing postnatal mouse. The globular AChE forms (G4, G2 and G1) were present in each cochlear turn at birth. An asymmetric AChE form (A12) was detected in the midturn on the 4th postnatal day, and in the base and apex on the 7th postnatal day. The activities of all AChE molecular forms increased rapidly during the second postnatal week and reached a plateau by the 19th postnatal day. In the 26-day old mouse, G4 constitutes the largest proportion of total cochlear AChE (57%), G2/G1 being 37% and A12 being 6%. The distribution of the AChE forms among the different turns is as follows: the combined value of the activities of G2 and G1 AChE was the same in each turn; G4 was the major form in the base and midturn; and A12, the least abundant AChE form of all, was localized mainly in the base. Our results indicate that in the cochlea (1) the content of molecular forms is similar to that of other neuronal systems, (2) the expression of AChE molecular forms is developmentally regulated, and (3) the AChE isoenzymes develop and are distributed differentially along the cochlear length; resulting near maturity in the greater proportional expression of G4 and A12 in the base and midturn and G2/G1 in the apex.