Properties of Exocytosis at the Rod Photoreceptor Terminal

Abstract

Abstract: : Purpose: To investigate the voltage-dependence, calcium-dependence and kinetics of exocytosis at the rod ribbon synapse. Methods: Capacitance changes were measured in enzymatically isolated rods using a two sine voltage stimulus protocol or a two-phase lock-in software amplifier. Intraterminal [Ca²⁺] was elevated by activation of voltage-gated Ca²⁺-channels or flash photolysis of caged-Ca²⁺ and monitored with Fura-FF. Results: Capacitance increases evoked by 100 ms depolarizing steps showed a sigmoidal relationship with voltage similar to the Ca²⁺ current (I_Ca)/voltage relationship and were Ca²⁺-dependent. Increasing the duration of a depolarizing step (-65 to -5 mV) from 12 ms to 2 s increased the amplitude of the capacitance jump until a plateau was reached. The relationship between capacitance and step duration was best fit by two exponentials with time constants of 4.4 and 303 ms. The plateau amplitude corresponded to the fusion of ~4500 vesicles, assuming a vesicle diameter of 45 nm, and is similar to the total number of vesicles estimated from electron microscopy to be tethered to the synaptic ribbons of a single salamander rod terminal. Consistent with previous findings (Rieke & Schwartz, 1996), exocytosis was triggered by [Ca²⁺]_i below 1 µM. Furthermore, over the physiological range of [Ca²⁺]_i of 0.5 to 3 µM, flash photolysis experiments showed that the rate of release was linearly related to [Ca²⁺]_i. Conclusions: The bi-exponential distribution of capacitance as a function of step duration suggests that rod photoreceptors, like goldfish Mb bipolar cells (Mennerick & Matthews, 1996), have two kinetically distinct phases of release. In addition, the correspondence between the number of tethered vesicles and the size of the releasable pool suggests that as in bipolar cells, the entire pool of ribbon-associated vesicles may be released. The linear relationship between I_Ca and capacitance is consistent with evidence for linearity between Ca²⁺ influx and exocytosis derived from other experimental approaches (Witkovsky et al., 1997). This linearity can be accounted for by the results of flash photolysis experiments which show a linear relationship between [Ca²⁺] and release rates over the physiological [Ca²⁺] range. This early linearity helps preserve response linearity across the photoreceptor synapse. Supported by EY10542, EY12128, and Research to Prevent Blindness.

Keywords: synapse • photoreceptors • electrophysiology: non-clinical