Feed-back modulation of cone synapses by L-horizontal cells of turtle retina.

Light stimulation of the periphery of the receptive field of turtle cones can evoke both transient and sustained increases of the cone Ca2+ conductance, which may become regenerative. Such increase in the cone Ca2+ conductance evoked by peripheral illumination results from the activation of a polysynaptic pathway involving a feed-back connexion from the L-horizontal cells (L-HC) to the cones. Thus the hyperpolarization of a L-HC by inward current injection can evoke a Ca2+ conductance increase in neighbouring cones. The cone Ca2+ channels thus activated are likely located at its synaptic endings and probably intervene in the cone transmitter release. Therefore the feed-back connexion between L-HC and cones by modifying the Ca2+ conductance of cones could actually modulate the transmitter release from cone synapses. Such feed-back modulation of cone synapses plays a role in the organization of the colour-coded responses of the chromaticity type-horizontal cells and probably of other second order neurones, post-synaptic to the cones. The mechanisms operating the feed-back connexion from L-HC to cones are discussed.