Distribution of frequenin in the mouse inner ear during development, comparison with other calcium-binding proteins and synaptophysin

Frequenin is a calcium-binding protein previously implicated in the regulat ion of neurotransmission. We report its immunocytochemical detection in the mouse inner ear, in the adult, and during embryonic (E) and postnatal (P) development. The distribution of frequenin was compared with those of other calcium-binding proteins (calbindin, calretinin, parvalbumin) and synaptop hysin. In the adult mouse inner ear, frequenin immunostaining was observed in the afferent neuronal systems (vestibular and cochlear neurons. their pr ocesses and endings) and in the vestibular and cochlear efferent nerve term inals. Frequenin colocalized with synaptophysin in well characterized presy naptic compartments, such as the vestibular and cochlear efferent endings, and in putative presynaptic compartments, such as the apical parr of the ve stibular calyces. Frequenin was not Found in vestibular hair cells and in c ochlear inner and outer hair cells. During development. frequenin immunorea ctivity was first detected on El I in rile neurons of the statoacoustic gan glion. On E14, frequenin was detected in the efferent neurites innervating the vestibular sensory epithelium, along with synaptophysin. On E16. freque nin was detected in the afferent neurites below the inner hair cells in the organ of Corti. The timing of frequenin detection in vestibular and cochle ar afferent neurites was consistent with their sequences of maturation, and was earlier than synaptogenesis. Thus ill the inner ear, frequenin is a ve ry early marker of differentiated and growing neurons and is present in pre synaptic and postsynaptic compartments. (C) 2000 Elsevier Science B.V. All rights reserved.