PATTERN OF SYNAPTOPHYSIN IMMUNOREACTIVITY IN THE EFFERENT NERVE-TERMINALS OF THE GUINEA-PIG COCHLEA

The goal of the present study was to analyze the distribution of effer ent 8th nerve synaptic endings in a surface preparation of the guinea pig cochlea using synaptophysin antibodies. Employing light and confoc al microscopy synaptophysin immunoreactivity was found exclusively at the base of the outer hair cells (OHCs) and the inner hair cells (IHCs ) axosomatic efferent synapses. Qualitative and quantitative differenc es were found between the OHCs and the IHCs immunoreactivity. Efferent nerve endings innervating IHCs were comparatively smaller, more numer ous and densely packed. Efferent terminals demonstrated a longitudinal gradient for the IHCs and a longitudinal and radial gradient for the OHCs. Quantitative analysis of synaptophysin immunofluorescence demons trated a higher percentage of efferent terminals innervating the IHCs and the OHCs in the mid and basal segments of the cochlea than in the apical regions. In addition, a radial gradient from the 1st to 3rd row of OHCs was evident. The results from the present study show that the analysis of synaptophysin immunoreactivity on cochlear surface prepar ations allows the efferent innervation to be determined throughout the entire cochlea. This technique allows for a rapid assessment of the n ormal cochlea as well as after cochlear insult. (C) 1997 Elsevier Scie nce Ireland Ltd.