title: 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 Ca2+ sensitivity or a lower dark concentration of Ca2+, we imaged fluorescent indicators of synaptic vesicle cycling and intraterminal Ca2+. We report that the Ca2+ sensitivity of release is indistinguishable in rods and cones, consistent with their possessing similar release machinery. However, the dark intraterminal Ca2+ concentration is lower in rods than in cones, as determined by two-photon Ca2+ imaging. The lower level of dark Ca2+ ensures that rods encode intensity with a slower vesicle release rate that is better matched to the lower information content of dim light.
authors.affiliation: Department of Molecular and Cell Biology, University of California, Berkeley, California 94720, USA.
authors.forename: Zejuan
authors.initials: Z
authors.lastname: Sheng
authors.forename: Sue-Yeon
authors.initials: SY
authors.lastname: Choi
authors.forename: Ajay
authors.initials: A
authors.lastname: Dharia
authors.forename: Jian
authors.initials: J
authors.lastname: Li
authors.forename: Peter
authors.initials: P
authors.lastname: Sterling
authors.forename: Richard H
authors.initials: RH
authors.lastname: Kramer
mesh_terms: D000557:Ambystoma / Q000033:anatomy & histology / Q000502:physiology; D000818:Animals; D002118:Calcium / Q000378:metabolism; D020013:Calcium Signaling / Q000502:physiology*; D003623:Dark Adaptation / Q000502:physiology*; D036681:Fluorescence Recovery After Photobleaching; D016257:Fura-2 / Q000031:analogs & derivatives / Q000494:pharmacology; D008116:Lizards / Q000033:anatomy & histology / Q000502:physiology; D046529:Microscopy, Electron, Transmission; D011726:Pyridinium Compounds / Q000378:metabolism; D000644:Quaternary Ammonium Compounds / Q000378:metabolism; D011930:Reaction Time; D017949:Retinal Cone Photoreceptor Cells / Q000378:metabolism* / Q000648:ultrastructure; D017948:Retinal Rod Photoreceptor Cells / Q000378:metabolism* / Q000648:ultrastructure; D013045:Species Specificity; D009435:Synaptic Transmission / Q000502:physiology*; D013572:Synaptic Vesicles / Q000378:metabolism* / Q000648:ultrastructure; D013997:Time Factors; D014785:Vision, Ocular / Q000502:physiology
chemical_list: C073804:FM1 43; D011726:Pyridinium Compounds; D000644:Quaternary Ammonium Compounds; C049925:fura-2-am; D002118:Calcium; D016257:Fura-2