226 | Probing AMPA receptors at ribbon synapses in the mammalian cochlea by glutamate-uncaging

Neural excitability, synaptic transmission and neuron-glia interactions

Author: Juan Goutman | Email: jgoutman@gmail.com

Juan Goutman , Mark Rutherford

2° Washington University – St. Louis

AMPA receptors in the mammalian brain mediate fast neurotransmission and are typically found in specific regions of the synapse called postsynaptic densities (PSDs). Across synapses a great variability of PSDs sizes and number of AMPA receptors has been described. This morphological heterogeneity is determinant of a variability of functional responses in the postsynaptic neuron. In the mammalian inner ear, glutamatergic synapses are also formed between inner hair cells (IHCs) and spiral ganglion neurons (SGNs). At these synapses all aspects of sound information are encoded and transmitted to the brain for further processing. A key aspect of the presynapse (IHCs) is the presence of a ‘synaptic body’ or ‘ribbon’ that concentrates large amounts of synaptic vesicles ensuring high rates of exocytotic events. Postsynaptic terminals of SGNs are characterized by large PSDs, 5 to 10 times bigger than those found in the brain. We speculate that PSDs are not saturated during normal neurotransmission and are able to accommodate large amounts of glutamate released by IHCs. To investigate this, we implemented a glutamate photolysis method by which a laser pulse is flashed upon the synapse previously bathed with a caged-glutamate compound, producing fast transients in glutamate concentration. Responses to glutamate uncaging were recorded by patch-clamp directly on SGNs terminals. Both the intensity and the duration of laser pulses could be modulated to generate transients of different sizes