Functional development of the medial olivocochlear efferent innervation before the onset of hearing

ZORRILA DE SAN MARTIN J; ALVAREZ HEDU¨¢N F; ELGOYHEN AB; KATZ E

Congreso; 34th Midwinter Meeting, Association for Research in Otolaryngology; 2012

Association for Research in Otolaryngology

Before the onset of hearing (postnatal day (P)12 in mice), inner hair cells (IHCs) are transiently innervated by medial olivocochlear (MOC) fibres. At P9-11 transmitter release is supported by both P/Q and N-type voltage-gated Ca2+ channels (VGCC) and negatively regulated by L-type VGCC, coupled to the activation of BK channels. We have previously shown that the quantal content (m) of transmitter release significantly increased between P5-7 and P9-11 (0.5 and 1.7, respectively) and that this increment was accompanied by dramatic changes in the short term plasticity (STP) properties of this synapse. Our goal is now to determine the basis for these developmental changes in synaptic transmission. Postsynaptic responses were monitored in whole-cell voltage-clamped IHCs while electrically stimulating the efferent fibres in isolated mouse organs of Corti. At P5-7, w-Agatoxin IVA reduced m to 37±6% of control (p<0.01; n=5 IHC) whereas 1 mM w-Conotoxin GVIA failed to block transmitter release (n=5 IHC), revealing that P/Q- but not N-type VGCC partially support transmitter release at the MOC-IHC synapse at this stage. To test whether BK channels modulate transmitter release, P5-7 cochleas were incubated with 100 nM Iberiotoxin, a specific BK channel antagonist. As reported for P9-11, m increased to 192±11 % of control (p<0.005, n=5 IHC). Since m depends both on the probability of each vesicle to be released and on the number of ready releasable vesicles, we estimated the readily releasable pool (RRP) size by repetitive stimulation protocols. The RRP size of the MOC-IHC synapse significantly increased from 4.7±1.0 vesicles at P5-7 to 10.7±2.3 vesicles at P9-11 (n=7 and n=10 IHC, respectively; p<0.01). Our results suggest that both differences in subtypes of VGCC that support transmitter release as well as differences in the RRP size, underlie the observed developmental changes in synaptic transmission. Support: CONICET, UBA, ANPCyT to ABE, EK, HHMI to ABE, NIHRO1 to PAF, ABE