Calcium Channels That Support Acetylcholine Release At the Transient

ZORRILLA DE SAN MARTIN, J.; WEDEMEYER , C; FUCHS, P.; ELGOYHEN, A.B; KATZ, E.

New London, NH, Estados Unidos

Conferencia; Gordon Conference on Auditory Systems; 2008

Gordon Research Conferences

Before the onset of hearing, inner hair cells (IHC) of the mammalian cochlea are transiently innervated by efferent fibers from the medial olivocochlear system. This synapse is cholinergic, inhibitory and mediated by the á9á10 nicotinic receptor. During postnatal development, IHC undergo dramatic changes in cholinergic sensitivity, pattern of innervation and expression of postsynaptic proteins. In developing synapses, synaptic modifications likely take place concurrently in both postsynaptic cells and presynaptic terminals. In mammals, fast synaptic transmission is mediated by multiple types of voltagegated Ca2+ channels (VGCCs), including N-, P/Q- and R-type. In this work, we evaluated the type/s of Ca2+ channels coupled to ACh release at the transient efferent-IHC synapse. We used an acutely isolated cochlear preparation from mice and studied the effects of different Ca2+ channel blockers on the quantal content (m) of transmitter release and on the amplitude of spontaneous synaptic currents (sIPSCs). Postsynaptic cholinergic currents (eIPSCs) in IHCs, voltage-clamped at -90 mV, were evoked by electrically stimulating the efferent fibers. Both w-conotoxinGVIA (300 nM), an N-type VGCC blocker and w- AgatoxinIVA (200 nM), a P/Q-type VGCC blocker, significantly reduced m to 44.5 ± 6.8 % (n=5) and 55.8 ± 15.1 % (n=4) of control, respectively. When both toxins were applied together, ACh release was reduced to 3.9 ± 0.6 of control (n=4). Conversely, m increased to 209 ± 29 % (n=5) of control in the presence of the L-type channel antagonist, Nifedipine (3 ìM). ACh release was also significantly enhanced (244 ± 41% of control, n= 7) by Iberiotoxin, a specific, BK channel antagonist. Preliminary results indicate that the block of BK channels occludes the enhancing effect of Nifedipine. None of the toxins tested affected the amplitude of sIPSCs, indicating that their site of action is only presynaptic. Our results suggest that the entry of Ca2+ through both N and P/Q-type VGCC support ACh release at this synapse, whereas the entry of Ca2+ through L-type channels might be activatating BK channels, which contribute to the repolarization of the presynaptic membrane, thus reducing the amount of ACh released per nerve impulse.