Synaptic transmission to the horizontal cells in the retina of the larval tiger salamander.

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Abstract
The receptive field diameter for most horizontal cells far exceeds the lateral spread of processes for any cell in larval Ambystoma mexicanum. Therefore horizontal cells probably receive synaptic input from neighbors and from the photoreceptors. The electrical effects of these 2 synaptic inputs were studied. The electrical properties of the horizontal cell inputs were characterized by determining the current-voltage curves in dark and light. These curves were compared with those obtained in the presence of blockers of synaptic transmission, and during the application of some putative transmitter agents. The light response of the cells with narrow receptive fields, near 0.5 mm in diameter, was associated with a conductance decrease. The input resistance was typically 25 M.OMEGA. and increased to about 33 M.OMEGA. during the light response. The current-voltage curves in light and dark intersected near + 50 mV. The current-voltage curve in the presence of Co2+ or Mg2+ was nearly identical to the curve in the light. The putative transmitter substances glutamate, aspartate and GABA depolarized the cells by increasing conductance. Current-voltage curves measured in the presence of these substances intersected the dark and light curves at + 50 mV, the same level at which the dark and light curves intersect. The light response of cells with broad receptive fields, between 1.0-2.0 mm, showed little or no change in conductance associated with the light response. The input resistance was near 20 M.OMEGA. and the current-voltage curves intersected at an extrapolated potential level near 200 mV. In the presence of ACh [acetylcholine], electrical properties of the broad field cells reverted to those of the narrow field cells: the receptive field was reduced to 0.5 mm, the input resistance increased, and the current-voltage curves intersected near + 50 mV. ACh appeared to interrupt synaptic input from neighboring horizontal cells. Horizontal cells receive a tonic excitatory input from the photoreceptors which is decreased by light. Horizontal cells receive an additional input from their neighbors is not associated with a measurable conductance change. The input from neighbors is selectively interrupted by ACh, but the nature of this synapse and the cholinergic action is unknown.