Gain of rod to horizontal cell synaptic transfer: relation to glutamate release and a dihydropyridine-sensitive calcium current.
We related rod to horizontal cell synaptic transfer to glutamate release by rods. Simultaneous intracellular records were obtained from dark-adapted rod-horizontal cell pairs. Steady-state synaptic gain (defined as the ratio of horizontal cell voltage to rod voltage evoked by the same light stimulus) was 3.35 +/- 0.60 for dim flashes and 1.50 +/- 0.03 for bright flashes. Under conditions of maintained illumination, there was a measurable increment of horizontal cell hyperpolarization for each light-induced increment of rod hyperpolarization over the full range of rod voltages. In separate experiments we studied glutamate release from an intact, light-responsive photoreceptor layer, from which inner retinal layers were removed. Steady light reduced glutamate release as a monotonic function of intensity; spectral sensitivity measures indicated that we monitored glutamate release from rods. The dependence of glutamate release on rod voltage was well fit by the activation function for a high-voltage-activated, dihydropyridine-sensitive L-type calcium current, suggesting a linear dependence of glutamate release on [Ca]i in the synaptic terminal. A simple model incorporating this assumption accounts for the steady-state gain of the rod to horizontal cell synapse.
Witkovsky
P
P
Department of Ophthalmology, New York University Medical Center, New York, New York 10016, USA.
Schmitz
Y
Y
Akopian
A
A
Krizaj
D
D
Tranchina
D
D
D000818:Animals; D002118:Calcium / Q000502:physiology*; D004095:Dihydropyridines / Q000494:pharmacology*; D004553:Electric Conductivity; D018698:Glutamic Acid / Q000378:metabolism*; D008027:Light; D008297:Male; D008959:Models, Neurological; D010786:Photoreceptor Cells / Q000502:physiology*; D017948:Retinal Rod Photoreceptor Cells / Q000378:metabolism / Q000502:physiology* / Q000528:radiation effects; D013569:Synapses / Q000502:physiology*; D014982:Xenopus laevis
D004095:Dihydropyridines; D018698:Glutamic Acid; D002118:Calcium