Immunocytochemical detection of calcitonin gene-related peptide in the postnatal developing rat cochlea

Using the immunofluorescence technique, three populations of fibers staining for calcitonin gene-related peptide are identified in postnatal developing cochleas and in adult rat cochleas. During the maturation of the cochlea, the immunostaining first appears in the basal turn and then extends toward the apex of the cochlea. The first population of immunostained fibers belongs to the lateral olivocochlear innervation. It is observed at postnatal day 4 within the inner spiral bundle of the organ of Corti. The second population of calcitonin gene-related peptide-like immunostained fibers belongs to the medial olivocochlear innervation. It is first identified at postnatal day 6 under the outer hair cells, generally in the first row. In the older stages, this population of fibers progressively extends toward the external row of outer hair cells. Finally, the third population of immunostained fibers belongs to the sympathetic supply of the cochlea. They can be identified at postnatal day 6 around the cochlear artery and its branches, and also within the entire modiolus. Our results confirm the presence of calcitonin gene-related peptide in fibers of the lateral and medial efferent innervations of the cochlea. They indicate an early appearance of calcitonin gene-related peptide in the developing cochlea, before the onset of the cochlear function, suggesting for this peptide either a neurotrophic function or a regulation of the early cochlear potentials at the level of the lateral efferent synapses.