Synaptic mechanisms involved in responses of chromaticity horizontal cells of turtle retina

Abstract

Chromaticity-type horizontal cells^1,2 are interneurones intervening in the first steps of colour-coding information in the vertebrate retina. In turtle retina, the most commonly found of these cells are the red/green horizontal cells (R/G–HCs)³ which are hyperpolarised by green light stimuli and depolarised by red ones. The hyperpolarisation of the R/G–HCs by green light results from a direct input from green cones which are hyper-polarised by green light⁴. The depolarisation of R/G–HCs by red light has been interpreted by Fuortes and Simon⁴ as a consequence of the depolarisation of the green cones due to the activation of a polysynaptic circuit (red cones to L-horizontal cells (L–HCs) to green cones) involving a negative feedback effect of L–HCs on the green cones. Feedback depolarisations of cones result from an increase of the cone calcium conductance which may become regenerative and give rise to spikes^6,7. Moreover, prolonged activation of the feedback mechanism of L–HCs on cones in the presence of Sr²⁺ ions in the extracellular medium evokes a repetitive discharge of spikes in cones⁸. In the present work, the effect of Sr²⁺ ions on the responses of both green cones and R/G–HCs have been analysed to test the Fuortes and Simon hypothesis. We have found that in Sr²⁺ media, red light stimuli covering a large retinal area elicit both a repetitive discharge of spikes in the green cones and trains of depolarising transient potentials, probably postsynaptic potentials, in the R/G–HCs. Such results support the idea that feedback depolarisation of the green cones is responsible for the depolarising responses of R/G–HCs to red light.