Differential sensitivity in the guinea pig olivo-cochlear bundle

The hair cells in the basal region of the cochlea are more susceptible to damage from ototoxic agents than are those of the rest of the cochlea, a result suggesting a chemical difference between the regions. We have recently obtained experimental evidence for a similar chemical differential in the cochlear efferents. Attempts to label efferent neurons with HRP by perfusion of scala tympani through holes in the apex and basal turn were uniformly negative, whereas perfusion via the round window labeled cell bodies in the superior olivary region. It was not necessary to introduce the perfusate via the round window; the previously unsuccessful perfusion via the basal hole became successful as soon as the round window was opened. We have been unable to obtain any satisfactory explanation of this phenomenon in terms of washing out of the perfusate or of different anatomical distributions of the fibers, and are driven to the conclusion that there is some chemical difference between the efferent terminals which results in the HRP being taken up only by those endings in the lower basal and “hook” regions.