Melatonin and its metabolizing enzymes in the lateral wall of the cochlea

Melatonin is a component of the protective mechanism of the cochlea and is formed from serotonin by the enzymes serotonin-N-acetyltransferase (AANAT) and hydroxyindole-O-methyltransferase (HIOMT). In synthesis, the intermediate N-acetyl-serotonin (NAS) is formed. The aim of this study is to characterize melatonin synthesis in the lateral wall of the cochlea, by histological detection of melatonin producing enzymes. In the present study, the expression patterns of melatonin and metabolizing enzymes (AANAT, HIOMT) were analyzed immunohistochemically in rats of two ages (postnatal, adult). The localization in stria vascularis and spiral ligament was determined. In adult animals, melatonin and the enzymes AANAT and HIOMT were detected in type I and type V fibrocytes the spiral ligament. The extracellular interspace of stria vascularis and areas of the spiral ligament showed AANAT but no HIOMT expression. In postnatal rats, the enzymes for melatonin synthesis were detected at the border between the spiral ligament and the stria vascularis as well as in the extracellular interspace of the stria vascularis. The boundary of the spiral ligament to the otic capsule expressed AANAT and melatonin. The melatonin synthesis can be assumed by co-localization of the enzymes AANAT and HIOMT which was found in the adult spiral ligament as well in postnatal but in smaller amounts. The expression of these markers in cells of the cochlear potassium circulation, may indicate the relevance of melatonin in process protection, which does not yet occur in postnatal animals. It can be assumed that NAS was synthesized in AANAT positive areas that were negative for HIOMT expression. The effect of this agonist for tropomyosin receptor kinase B needs further research in the cochlea.

Publication History

Article published online:
12 May 2023