188 Brain Research, 220 (1981) 188-193 Elsevier/North-Holland Biomedical Press Presynaptic bodies in outer hair cells of the chinchilla organ of Corti JONATHAN H. SIEGEL and WILLIAM E. BROWNELL Department of Physiology and Biophysics, Washington University Medical School, Box 8101, St. Louis, Mo. 63110 and Departments of Neuroscience and Surgery (ENT), University of Florida, Gainesville, Fla. 32610 (U.S.A.) (Accepted April 23rd, 1981) Key words: cochlea - - hair cells - - presynaptic bodies - - synaptic vesicles Some, but not all synapsesof chinchilla outer hair cells with terminals of the spiral afferent nerve fibers have presynaptic bodies which are often surrounded by a halo of synaptic vesicles. The variety of appearance of presynaptic bodies and vesicles may result from the conditions of fixation or may represent differences in functional state. Electron-dense presynaptic bodies associated with synaptic vesicles have been reported opposite afferent endings in the receptor cells of a variety of sensory systems: retinal photoreceptors2,17, is, lateral line electroreceptors and mechanoreceptors 7,10,14, vestibular hair cells4, 25 and in cochlear hair cells 2°. In addition to the presence of presynaptic bodies at their afferent terminals these cells share a number of other features: modulation of neurotransmitter release is associated with receptor potentials reported to be less than 20 mV; the regenerative properties that lead to Na+-K ~- mediated action potentials have not been associated with their cytoplasmic mem- branes; and with the exception of electroreceptors they possess ciliated structures or their remnants. Presynaptic bodies have also been observed in retinal bipolar cells 12. Similar structures aggregate in 'ribbon fields' in pinealocyte cells 9. Retinal bipolar cells and pinealocytes resemble the above-mentioned receptor cells in that they are electrically inexcitable. Of particular interest is the observation that while presynaptic bodies are consistently present in mammalian cochlear inner hair cells 3,19 they have been reported as being only rarely found in the outer hair cells of the chinchilla organ of Corti 19 and absent in the outer hair cells of the catZ, ~6. The apparent absence of accumulations of vesicles associated with synaptic bodies opposite afferent terminals in outer hair cells of cats and chinchillas has led to speculation that transmission at these synapses may be significantly different from that at other hair cell afferent synapses3, 6 or that the synapses are not functionalZL In chinchilla cochleas using the uptake of horseradish peroxidase by cochlear hair cells, we have observed numerous presynaptic bodies. A variation of the rapid Kalt and Tandled 1 fixative was used: 0.6 % acrolein, 2 % paraformaldehyde, 3 % glutaraldehyde, 0.6 % dimethylsulfoxide in 0.1 M sodium