SUMMARY1. By applying atomized chemical solutions on to gecko and carp retinae, neuropharmacological reactions of the photoreceptors and horizontal cells were observed.2. Sodium L-glutamate and L-aspartate, glycine, ACh and GABA, had no appreciable effect on the photoreceptor activity.3. Carp horizontal cells were depolarized by both L-glutamate and Laspartate. When maximally depolarized, the action of the endogenous transmitter from the receptor terminals was completely masked, resulting in abolition of the S-potentials.4. Responses to red light in both L-and C-type horizontal cells were more strongly affected by aspartate than responses to blue light.5. Glycine and GABA hyperpolarized the horizontal cells, and the Spotentials were diminished. 6. ACh had no effect on the activity of the horizontal cells.
The synaptic contacts between photoreceptors and horizontal cells in the retina of the turtle (Geoclemys) were studied. Horizontal cells were classified into three types according to their intracellularly recorded spectral responses: luminosity, biphasic chromaticity, and triphasic chromaticity horizontal cells (LHC, BHC, and THC). These cells were then iontophoretically filled with horseradish peroxidase (HRP). The various types of photoreceptors located within the dendritic field of the HRP-filled horizontal cells were identified either as rods or as one of the three chromatic types of cones; the latter were identified by the presence and colors of their oil droplets in the inner segments. The synaptic contacts between photoreceptors and labeled horizontal cells were then investigated by light and electron microscopy of serial sections on three LHCs, two BHCs, and one THC. LHCs made synaptic contacts with about 100 photoreceptors, including rods and three chromatic types of cones; two-thirds of these photoreceptors contacted the cell body and the remaining its axon terminal. BHCs contacted about 30 cones; red-, green-, and blue-sensitive cones in the ratio of 3:4:1. THC contacted 20 cones; red- and blue-sensitive cones in the ratio of 2:1. The dendritic processes of HRP-filled horizontal cells were found as the lateral element of the ribbon synaptic complexes. The present finding suggests that the responses of BHC and THC to red flashes are fed directly from red-sensitive cones in addition to the feedback pathway of cone-horizontal cell connections in the previous studies.
The spectral sensitivities and color of oil droplets of cone photoreceptors in the retina of the red-eared turtle (Pseudemys scripta elegans) were investigated by intracellular recording and injections of Lucifer yellow dye. Six morphological types of cones could be distinguished by the color of the oil droplets located in the outermost inner segments. Single cones containing either red or pale green oil droplets were sensitive to red light, cones with yellow oil droplets to green, and cones with clear oil droplets to blue. Contrary to previous reports, both principal and accessory members of double cones were sensitive to red, and no diffusion of dye was detected between the two apposed members. Thus, the oil droplets provide a reliable morphological basis for further investigation of the neuroanatomical networks underlying the processing of color information in the vertebrate retina.
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