C-type natriuretic peptide-like immunoreactivity in the rat inner ear.

C-type natriuretic peptide (CNP) is a member of the atrial natriuretic peptide family (ANP family). The family also includes ANP and brain natriuretic peptide (BNP). These peptides regulate the homeostasis of body fluid and blood pressure as a neuropeptide in the central nervous system as well as a cardiac hormone in the periphery. We have recently reported the expression of CNP mRNA in the inner ear. To assess the possible physiological role of CNP in the inner ear, we investigated the localization of CNP peptide in the rat inner ear by immunohistochemistry at the light and electron microscopic level. CNP-like immunoreactivity was widely distributed in the secretory and the neuronal portion of the inner ear, i.e. the spiral ligament, the dark cell region of the utriculus, the epithelium of the endolymphatic sac, the spiral ganglion cells and the vestibular ganglion cells. The results suggest that CNP may play a role in the homeostasis of the perilymph and endolymph and may also influence nerve activities in the inner ear.