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

San Diego

Congreso; 35th Midwinter Meeting of the 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 innervated by medial olivocochlear (MOC) fibers. 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, functionally coupled to the activation of BK channels (Zorrilla de San Martín et al., 2010). In previous work we showed 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 present goal is 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 fibers in isolated mouse organs of Corti. At P5-7, w-Agatoxin IVA reduced m to 37±6% of control (p<0.01; n=5 cells) whereas 1 mM w-Conotoxin GVIA failed to block transmitter release (n=5 cells), 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 cells). In addition, we estimated the readily releasable pool (RRP) size by repetitive stimulation protocols applied to the MOC fibers. 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 cells, respectively; p<0.01). Our results suggest that both differences in the subtypes of VGCC that support transmitter release as well as differences in the RRP size, underlie the observed developmental changes in synaptic transmission.