Both high- and low voltage-activated calcium currents contribute to the light-evoked responses of luminosity horizontal cells in the Xenopus retina.
We examined the contribution of two intrinsic voltage-dependent calcium channels to the light-evoked responses of a non-spiking retinal neuron, the horizontal cell (HC). HC's isolated from the Xenopus retina were studied by the whole cell version of the patch clamp. In a mixture of agents which suppressed Na- and K-dependent currents, we identified a transient, low voltage-activated Ca current suppressed by Ba2+ and blocked by Ni2+ (T-type) and a sustained, high voltage-activated, dihydropyridine-sensitive Ca current that was enhanced by Ba2+ (L-type). We made simultaneous intracellular recordings from rods and HC's in the intact, dark-adapted Xenopus retina. Under certain stimulus conditions, transient oscillations appeared in HC responses but were absent in rod light-evoked waveforms. One type of transient was seen at relatively hyperpolarized potentials (< -45 mV), was enhanced by Sr2+ and inhibited by Ni2+. It thus appears to depend on a T-type Ca-current. A second type of oscillation was seen to be superimposed on a prolonged depolarizing wave following light off in the HC and as spike-like depolarizations in rods. These oscillations were enhanced by Ba2+ and Sr2+, but blocked by the dihydropyridine, nifedipine, indicating their dependence on an L-type calcium conductance. All calcium-dependent oscillations were suppressed by 0.05-0.5 mM Co2+. Suppression of glutamate neurotransmission with CNQX or kynurenate, or glycine neurotransmission with strychnine, enhanced the HC oscillations.
Akopian
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A
Department of Ophthalmology, New York University Medical Center, NY 10016, USA.
Krizaj
D
D
Witkovsky
P
P
D018750:6-Cyano-7-nitroquinoxaline-2,3-dione / Q000494:pharmacology; D000221:Adaptation, Ocular / Q000502:physiology*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000502:physiology*; D004594:Electrophysiology; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D018698:Glutamic Acid / Q000494:pharmacology; D005998:Glycine / Q000494:pharmacology; D018684:Glycine Agents / Q000494:pharmacology; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology*; D008024:Ligands; D008297:Male; D008564:Membrane Potentials / Q000502:physiology; D010507:Periodicity; D010775:Photic Stimulation; D012160:Retina / Q000737:chemistry / Q000166:cytology* / Q000502:physiology; D013268:Stimulation, Chemical; D013331:Strychnine / Q000494:pharmacology; D014982:Xenopus laevis
D015220:Calcium Channels; D018691:Excitatory Amino Acid Antagonists; D018684:Glycine Agents; D008024:Ligands; D018698:Glutamic Acid; D018750:6-Cyano-7-nitroquinoxaline-2,3-dione; D013331:Strychnine; D002118:Calcium; D005998:Glycine