CGRP-like immunoreactivity in the guinea pig organ of Corti: a light and electron microscopy study

We examined calcitonin-gene related peptide (CGRP)-like immunoreactivity in the guinea pig organ of Corti at both the light and electron microscope level using immunofluorescence and immunoperoxidase techniques. We observed strong CGRP-like immunoreactivity in the inner spiral bundle and tunnel spiral bundle in all turns at both the light and electron microscope level. CGRP immunostaining was localized exclusively in vesiculated efferent fibers. The majority of these fibers in the inner spiral bundle were immunoreactive. We observed immunostained synaptic junctions between two efferent fibers and between efferent varicosities and afferent dendrites. In the region of the outer hair cells (OHC), we found CGRP-like immunoreactivity in the efferent endings at the base of OHCs and at the level of the OHC nucleus. At the base of OHCs, CGRP reactive endings in the first, second and more rarely the third row. CGRP immunoreactivity was localized in large dense core vesicles. In summary, our data demonstrate that the neuropeptide, CGRP, was found in both the lateral and medial efferent systems. Secondly, the immunoreactivity was localized in large dense core vesicles.