Upregulated Expression and Shifted Distribution of Melatonin and Its Synthesizing Enzymes From Postnatal to Young Adult Rat Cochleae

The cochlea's cellular architecture plays a critical role in auditory perception, yet is highly susceptible to degenerative factors. While melatonin is known for its antioxidative properties in the adult cochlea, its expression during early development remains understudied. This study used immunohistochemical staining of melatonin and its synthesizing enzymes (AANAT, HIOMT) to explore the self-synthesis and spatial distribution of melatonin in the cochlea of postnatal and adult rats. Postnatal rats exhibited low levels of intracellular marker expression in the lateral wall and the undifferentiated sensory epithelia, with no expression observed in the spiral ganglion. They showed mainly extracellular marker expression near the stria vascularis, in the stria vascularis interspace, and above undifferentiated sensory epithelia. In contrast, adults exhibited widespread intracellular marker presence in spiral ligament fibrocytes, spiral ganglion neurons, satellite glia, and epithelial supporter cells, except in hair cells. Fibrocytes in the spiral limbus expressed AANAT and HIOMT at both developmental stages. The present findings indicate melatonin's complex involvement in cochlear protection and development. Detailed knowledge of melatonin synthesis locations and intensity across different age stages within the auditory system holds the key to pioneering novel treatments, preventive strategies, and a deeper understanding of hearing processes.