Biomedical subjects

Jonathan F Ashmore

Publications and source records attributed to Jonathan F Ashmore.

3 recordsLinked to original sources

PubMed2005

Fast vesicle replenishment allows indefatigable signalling at the first auditory synapse.

Ribbon-type synapses in inner hair cells of the mammalian cochlea encode the complexity of auditory signals by fast and tonic release through fusion of neurotransmitter-containing vesicles. At any instant, only about 100 vesicles are tethered to the synaptic ribbon, and about 14 of these are docked to the plasma membrane, constituting the readily releasable pool. Although this pool contains about the same number of vesicles as that of conventional synapses, ribbon release sites operate at rates of about two orders of magnitude higher and with submillisecond precision. How these sites replenish their vesicles so efficiently remains unclear. We show here, using two-photon imaging of single release sites in the intact cochlea, that preformed vesicles derived from cytoplasmic vesicle-generating compartments participate in fast release and replenishment. Vesicles were released at a maximal initial rate of 3 per millisecond during a depolarizing pulse, and were replenished at a rate of 1.9 per millisecond. We propose that such rapid resupply of vesicles enables temporally precise and sustained release rates. This may explain how the first auditory synapse can encode with indefatigable precision without having to rely on the slow, local endocytic vesicle cycle.

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PubMed2002

Fm1-43 reveals membrane recycling in adult inner hair cells of the mammalian cochlea.

Neural transmission of complex sounds demands fast and sustained rates of synaptic release from the primary cochlear receptors, the inner hair cells (IHCs). The cells therefore require efficient membrane recycling. Using two-photon imaging of the membrane marker FM1-43 in the intact sensory epithelium within the cochlear bone of the adult guinea pig, we show that IHCs possess fast calcium-dependent membrane uptake at their apical pole. FM1-43 did not permeate through the stereocilial mechanotransducer channel because uptake kinetics were neither changed by the blockers dihydrostreptomycin and d-tubocurarine nor by treatment of the apical membrane with BAPTA, known to disrupt mechanotransduction. Moreover, the fluid phase marker Lucifer Yellow produced a similar labeling pattern to FM1-43, consistent with FM1-43 uptake via endocytosis. We estimate the membrane retrieval rate at approximately 0.5% of the surface area of the cell per second. Labeled membrane was rapidly transported to the base of IHCs by kinesin-dependent trafficking and accumulated in structures that resembled synaptic release sites. Using confocal imaging of FM1-43 in excised strips of the organ of Corti, we show that the time constants of fluorescence decay at the basolateral pole of IHCs and apical endocytosis were increased after depolarization of IHCs with 40 mm potassium, a stimulus that triggers calcium influx and increases synaptic release. Blocking calcium channels with either cadmium or nimodipine during depolarization abolished the rate increase of apical endocytosis. We suggest that IHCs use fast calcium-dependent apical endocytosis for activity-associated replenishment of synaptic membrane.

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PubMedDate not supplied

Cochlear transduction: from models to molecules and back again.

The strides made over the last few years towards understanding many details of cochlear function still leave a number of issues unresolved. Integrating the information from molecular, genetic and, increasingly, genomic sources requires models that provide close matching between data and theory. For both theoretical and experimental reasons, the difficult area in cochlear physiology has been to understand how sensory transduction operates at the basal end of the mammalian cochlea. The identification of candidate motor proteins in outer hair cells (OHCs) draws attention to the question of whether we understand cochlear tuning. Nevertheless, the association of the cloned motor protein 'prestin' with an anion transporter superfamily provides clues about the molecular nature of the OHC motor in the basolateral membrane, the utilisation of chloride in hair cells and the long-term stability of small basal turn cochlear hair cells.

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