[Abstract Title]. - Society for Neuroscience

Topic: D.02.a. Mechanoreceptors and cochlea
Support: BMBF Grant to TM and AN
DFG Fellowship to NS
Jack Kent Cooke Foundation
NIDCD Grant P30 DC 00529 to MCL
NIDCD Grant RO1 DC 00188 to MCL
Title: Synaptic ribbons in cochlear hair cells are required for reliable coding of stimulus onset
Authors: *B. N. BURAN 1,2 , N. STRENZKE 3,2 , A. NEEF 4 , T. MOSER 3,4 , M. C.
LIBERMAN 1,5,2 ;
1 Div. of Hlth. Sci. and Technol., Harvard-MIT, Cambridge, MA; 2 Eaton-Peabody Lab.,
Massachusetts Eye and Ear Infirmary, Boston, MA; 3 Inner Ear Lab, Dept. of Otolaryngology,
4 Bernstein Ctr. for Computat. Neurosci., Univ. of Goettingen, Goettingen, Germany; 5 Dept. of
Otology and Laryngology, Harvard Med. Sch., Boston, MA
Abstract: Synaptic ribbons are electron-dense structures surrounded by vesicles and tethered to
the presynaptic membrane of photoreceptors, retinal bipolar cells and hair cells. Such synapses
are characterized by sustained exocytosis that is graded with stimulus intensity and can achieve
very high rates of release.
To gain insight into the function of this specialized presynaptic machinery, we characterized the
response properties of single auditory nerve (AN) fibers in a mouse with targeted deletion of
bassoon, the gene for a presynaptic scaffolding protein, in which synaptic ribbons are no longer
tethered to the active zone. Prior in vitro studies showed a strong reduction of hair cell
exocytosis in these mutants. While informative, whole-cell responses reflect the summed activity
of many synapses, which are heterogeneous with respect to spontaneous and sound-evoked rates.
Since each mammalian AN fiber receives input from a single inner hair cell active zone, to
which, typically, a single ribbon is tethered, single-fiber recordings in bassoon mutants and
control mice offer a sensitive functional metric of the contribution of individual synaptic ribbons.
Response properties of mutant AN fibers were similar, in many respects, to wild-type. Spike
intervals retained their normal irregularity, thresholds were unaffected, dynamic range was
unchanged, spike synchronization to stimulus phase was unimpaired, post-onset adaptation and
recovery from adaptation were normal, and the ability to sustain discharge throughout a longduration
stimulus was unaffected. However, striking abnormalities were seen in absolute
discharge rates. Mean spontaneous rate was decreased by a factor of 4 and sound-evoked rates
were decreased by a factor of 2. A corresponding increase was observed in the variance of the
first spike latency in response to moderate level tone bursts. This increased jitter likely arises
from a reduction in synaptic drive through a mechanism such as decreased probability of
simultaneous vesicle release, and may degrade the ability of the auditory system to resolve
stimulus features such as interaural time differences that mediate sound localization and other
key aspects of binaural hearing.