Synaptic bodies in the different rows of outer hair cells in the guinea pig cochlea

The afferent synapses of the outer hair cells (OHCs) of the cochlea are peculiar, insofar as some of them contain special synaptic bodies (SBs) forming the so-called ribbon synapses. These SB-containing synapses are highly variable in number, exhibiting interspecies and intraspecies differences. As quantitative data on the incidence of SBs in the different rows of OHCs are lacking and as some of the above differences may have a circadian basis, in the present study SBs were counted in guinea pigs killed at different times of the day and night. In the second turn of the cochlea, synapses with perpendicular and parallel SBs were distinguished. Perpendicular SBs, but not parallel SBs, were significantly higher in number in the third row. Circadian changes were observed for perpendicular SBs, which were higher in number in the evening than in the morning. Circadian changes were absent in the parallel SBs of the OHCs and in the SBs of the inner hair cells. These results show that the afferent synapses of OHCs are rather complex, structurally as well as temporally; the functional significance of this finding remains to be elucidated.