POSTNATAL CHANGES IN COCHLEAR POLYAMINE METABOLISM IN THE RAT

Ornithine decarboxylase (ODC), the initial enzyme in the polyamine biosynthetic pathway, is increased in the developing rat cochlea, suggesting that polyamine biosynthesis is important in cochlear development. Although cochlear polyamines have been detected in adult rats, they have not been identified in developing rats. We quantified polyamines in the developing and mature rat cochlea and further characterized ODC in the early postnatal period. Putrescine and spermidine in combined tissues of the organ of Corti and lateral wall of the cochlea were highest during the first 10 postnatal days, then declined to adult levels shortly thereafter. Spermine demonstrated a similar developmental trend. A high spermidine to spermine ratio was noted during this period as was rapidly increasing ODC activity. A high spermidine/spermine ratio was also noted in the cochlear nerve of developing and mature rats, suggesting that spermidine may be necessary for function and maintenance of the nerve. This is the first report of polyamines in the developing rat cochlea. The period of increased polyamine synthesis coincides with the critical period for ototoxicity induced by cr-difluoromethylornithine, a specific ODC inhibitor, and the period of rapid cochlear development.