Localization and properties of voltage-gated calcium channels in cone photoreceptors of Tupaia belangeri.
In spiking neurons, phasic, calcium-dependent transmitter release is initiated when a presynaptic action potential activates voltage-dependent calcium channels. Vertebrate photoreceptors are nonspiking neurons that continuously release transmitter. This study uses patch-clamp recording to examine the electrophysiological properties of mammalian cones in intact retina. The cell capacitance was 10+/-1 pF and the input resistance was 0.52+/-0.46 G omega at -65 mV (31 cells). A specific membrane capacitance of 1.2 pF/cm2 was calculated. The cones did not appear to be chemically or electrically coupled. The calcium conductance averaged 3+/-1 nS (five cells). Fifty percent of the calcium channels were active at -40 mV, and at this voltage the number of active channels changed e-fold for a 6-mV voltage change. At 25 degrees C, the current reached a peak within about 1 ms after onset of a step to -35 mV. The calcium influx produced by depolarization activated a chloride conductance with a delay of a few milliseconds. The channels did not completely inactivate during maintained depolarization. The calcium channels were partially blocked by high concentrations of nifedipine, an L-type specific antagonist, and were recognized by an antibody raised against the L-type subunit alpha-1D. The immunohistochemical staining shows that the calcium channels are localized to the synaptic terminals. The immunohistochemical, physiological, and pharmacological properties indicate that the calcium channels in mammalian photoreceptors may represent a novel isoform, possibly with some homologies to the L-type class. The activation range of the channels matches the physiological operating range of photoreceptors.
Department of Neuroanatomy, Max-Planck-Institute for Brain Research, Frankfurt, Germany.
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D000818:Animals; D002118:Calcium / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism*; D018118:Chloride Channels / Q000378:metabolism; D002712:Chlorides / Q000378:metabolism; D005260:Female; D019084:Fluorescent Antibody Technique, Indirect; D008297:Male; D008564:Membrane Potentials / Q000502:physiology; D009543:Nifedipine / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D010786:Photoreceptor Cells / Q000378:metabolism; D017729:Presynaptic Terminals / Q000378:metabolism*; D017949:Retinal Cone Photoreceptor Cells / Q000502:physiology*; D014417:Tupaia
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D018118:Chloride Channels; D002712:Chlorides; D009543:Nifedipine; D002118:Calcium