That firing rates do not adapt to zero but rather to a relatively high rate indicates that trafficking (superlinear component) is rapidly accessible under physiological conditions. Similar to the response described in Figure 5, under physiological conditions the processes tend to merge but vesicle release shows a reduction in slope initially that becomes sustained. The level of neural adaptation may in part be determined by how rapidly each synapse is capable of recruiting

vesicles between pools—the faster the recruitment, BMN 673 research buy the less adaptation is observed. In fact, it may be argued that steady-state firing requires recruitment of vesicles such that the rate of release at any given synapse may be dictated by access to the reserve pool of vesicles. Thus it may be that spontaneous firing rates are regulated

by resting calcium currents and vesicles in the RRP and recycling pool, while stimulated release is more dependent upon vesicle recruitment from the reserve pool and the ability to modulate release of stored calcium (Guth et al., 1991). In summary, we used real-time capacitance measurements to identify saturable pools of vesicles and discovered a superlinear release component requiring recruitment of vesicles to release sites. We suggest that Ca2+-dependent vesicle trafficking CB-839 is responsible for this movement, which is required for hair cell synapses to maintain high rates of sustained vesicle fusion. We postulate that the superlinear release component reflects synapses operating at maximal rates of release and trafficking Histidine ammonia-lyase and that release of an as yet undefined internal pool of Ca2+ may be required. These characteristics of synaptic vesicle recruitment and release make hair cell ribbon

synapses quite unique as compared to other synapses. The auditory papilla from red-eared sliders (Trachemys scripta elegans) were prepared as previously described ( Schnee et al., 2005) by using methods approved by the IACUC committee at Stanford University and following standards established by NIH guidelines. Tectorial membranes were removed as previously described by using a hypertonic and hypercalcemic (10 mM Ca2+) solution ( Farris et al., 2006). The external recording solution contained 125 mM NaCl, 0.5 mM KCl, 2.8 mM CaCl2, 2.2 mM MgCl2, 2 mM pyruvate, 2 mM creatine, 2 mM ascorbate, 6 mM glucose, and 10 mM N-(2-hydroxy-ethyl) piperazine-N′-(2-ethanesulfonic acid) (HEPES) with pH adjusted to 7.6 and osmolality maintained at 275 mosml/kg. One hundred nanometers apamin was included in the external solution to block SK potassium currents ( Tucker and Fettiplace, 1996). Cells were imaged with a BX51 fixed-stage upright microscope (Olympus) with bright-field optics. Conventional epifluorescence was used for the Ca2+ imaging.