Abstract
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 GΩ 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°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 α-ID. 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.
Original language | English (US) |
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Pages (from-to) | 541-552 |
Number of pages | 12 |
Journal | Visual neuroscience |
Volume | 15 |
Issue number | 3 |
DOIs | |
State | Published - May 1998 |
Externally published | Yes |
Keywords
- Calcium channels
- Calcium-dependent chloride current
- Electrotonic properties
- Mammalian photoreceptor
ASJC Scopus subject areas
- Physiology
- Sensory Systems