Synaptic Ca²⁺in Darkness Is Lower in Rods than Cones, Causing Slower Tonic Release of Vesicles

Abstract

Rod and cone photoreceptors use specialized biochemistry to generate light responses that differ in their sensitivity and kinetics. However, it is unclear whether there are also synaptic differences that affect the transmission of visual information. Here, we report that in the dark, rods tonically release synaptic vesicles at a much slower rate than cones, as measured by the release of the fluorescent vesicle indicator FM1-43. To determine whether slower release results from a lower Ca 2ϩ sensitivity or a lower dark concentration of Ca 2ϩ , we imaged fluorescent indicators of synaptic vesicle cycling and intraterminal Ca 2ϩ . We report that the Ca 2ϩ sensitivity of release is indistinguishable in rods and cones, consistent with their possessing similar release machinery. However, the dark intraterminal Ca 2ϩ concentration is lower in rods than in cones, as determined by two-photon Ca 2ϩ imaging. The lower level of dark Ca 2ϩ ensures that rods encode intensity with a slower vesicle release rate that is better matched to the lower information content of dim light.

Phosphorylation of the Ca2+-Binding Protein CaBP4 by Protein Kinase C ζ in Photoreceptors

“…This difference in transmission may result from a higher intraterminal Ca 2ϩ concentration in cones compared with rods in the dark (Sheng et al, 2007). We have shown previously that CaBP4 is required for normal cone to bipolar cell transmission (Maeda et al, 2005).…”

“…Light-dependent protein phosphorylation is a common regulatory mechanism in photoreceptors (Kuhn and Dreyer, 1972;Lee et al, 1984;BoeszeBattaglia et al, 1997;Roof et al, 1997;Hayashi et al, 2000;Hu et al, 2001;Rajala et al, 2002;Trojan et al, 2003 (Morgans et al, 1998;Morgans, 1999;Barnes and Kelly, 2002), which show faster rates of neurotransmitter release compared with rods in darkness (Choi et al, 2005;Sheng et al, 2007). This difference in transmission may result from a higher intraterminal Ca 2ϩ concentration in cones compared with rods in the dark (Sheng et al, 2007).…”

Calmodulin enhances ribbon replenishment and shapes filtering of synaptic transmission by cone photoreceptors

“…Other factors such as vesicle descent down the ribbon may contribute to slower kinetics of replenishment, but if vesicle delivery to the ribbon is the rate-limiting step in replenishment, then this suggests that the probability of a single vesicle attaching to the ribbon upon collision is likely to be significantly <1. Although we used vesicle density measurements from salamander cones (Sheng et al, 2007), vesicles appear less concentrated in mouse rod terminals, 580-750 v/µm 3 (Zampighi et al, 2011), implying there may be cell-tocell or species-to-species differences in the kinetics of vesicle resupply.…”

“…To explore whether the rate of vesicle collisions is itself rate limiting, we first assumed that all ribbon collisions result in successful attachment (s = 1), yielding a lower bound on the replenishment time constant,  a . Using empirically determined values from photoreceptor synapses of D = 0.11 µm 2 /s,  = 2,210 v/µm 3 , and  = 45 nm (Rea et al, 2004;Thoreson et al, 2004;Sheng et al, 2007), we obtained  a = 91 ms. This is roughly an order of magnitude faster than the experimentally measured value for  a .…”

Light-Driven Calcium Signals in Mouse Cone Photoreceptors

“…In recent years, Ca 2ϩ signaling in photoreceptors was investigated in different vertebrate species, including amphibians (Sheng et al, 2007;Choi et al, 2008) and mammals (Johnson et al, 2007), using optical methods and AM-ester loading (Tsien, 1981) of synthetic fluorescent Ca 2ϩ indicators. While this approach provided valuable estimates of steady-state [Ca 2ϩ ] in rods and cones, AM-dye loading is problematic since it requires organic solvents (e.g., DMSO), it is strongly species-and age-dependent, and produces formaldehyde as a consequence of the intracellular AM-esters cleavage (Tsien et al, 1982;Rink and Pozzan, 1985).…”