title: Calcium spike-mediated digital signaling increases glutamate output at the visual threshold of retinal bipolar cells.
abstract: Most retinal bipolar cells (BCs) transmit visual input from photoreceptors to ganglion cells using graded potentials, but some also generate calcium or sodium spikes. Sodium spikes are thought to increase temporal precision of light-evoked BC signaling; however, the role of calcium spikes in BCs is not fully understood. Here we studied how calcium spikes and graded responses mediate neurotransmitter release from Mb-type BCs, known to produce both. In dark-adapted goldfish retinal slices, light induced spikes in 40% of the axon terminals of intact Mbs; in the rest, light generated graded responses. These light-evoked membrane potentials were used to depolarize axotomized Mb terminals where depolarization-evoked calcium current (ICa) and consequent exocytosis-associated membrane capacitance increases (ΔCm) could be precisely measured. When evoked by identical dim light intensities, spiking responses transferred more calcium (Q(Ca)) and triggered larger exocytosis with higher efficiency (ΔCm/Q(Ca)) than graded potentials. Q(Ca) was translated into exocytosis linearly when transferred with spikes and supralinearly when transferred with graded responses. At the Mb output (ΔCm), spiking responses coded light intensity with numbers and amplitude whereas graded responses coded with amplitude, duration, and steepness. Importantly, spiking responses saturated exocytosis within scotopic range but graded potentials did not. We propose that calcium spikes in Mbs increase signal input-output ratio by boosting Mb glutamate release at threshold intensities. Therefore, spiking Mb responses are suitable to transfer low-light-intensity signals to ganglion cells with higher gain, whereas graded potentials signal for light over a wider range of intensities at the Mb output.
authors.affiliation: Department of Biomedical Sciences, Colorado State University, Fort Collins, Colorado.
authors.forename: Mikhail Y
authors.initials: MY
authors.lastname: Lipin
authors.affiliation: Department of Biomedical Sciences, Colorado State University, Fort Collins, Colorado jozsef.vigh@colostate.edu.
authors.forename: Jozsef
authors.initials: J
authors.lastname: Vigh
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D020013:Calcium Signaling / Q000502:physiology*; D025802:Electric Capacitance; D005089:Exocytosis / Q000502:physiology; D018698:Glutamic Acid / Q000378:metabolism*; D006054:Goldfish; D016014:Linear Models; D008564:Membrane Potentials / Q000502:physiology; D018408:Patch-Clamp Techniques; D010775:Photic Stimulation; D017729:Presynaptic Terminals / Q000502:physiology; D051245:Retinal Bipolar Cells / Q000502:physiology*; D012684:Sensory Thresholds / Q000502:physiology*; D013997:Time Factors; D046509:Tissue Culture Techniques; D014785:Vision, Ocular / Q000502:physiology*
chemical_list: D018698:Glutamic Acid; D002118:Calcium
keywords: bipolar cell; calcium spike; graded potential; light response; threshold