Physiology of HI horizontal cells in the primate retina.

Two types of recording were obtained from horizontal cells in the superfused eyecup preparation of the retina of Macaca mulatta. Injection of horseradish peroxidase showed that one class of responses was generated by the cell body of H I horizontal cells. The origin of the other type of responses was not identified morphologically, but they were strikingly similar to those recorded from the axonal arborization of axon-bearing horizontal cells of other mammalian species. The soma of H I cells responded with graded hyperpolarizations to increasing changes in diffuse white-light illumination. Threshold responses were obtained with stimulus intensities above electroretinogram (ERG) b-wave threshold; this was taken as an indication of cone input to this part of the cell. Responses to low-intensity stimuli were small, sustained hyperpolarizations; at intermediate intensities, the hyperpolarizations were initiated by a prominent transient component and were followed by a small rod after-effect at stimulus termination; with bright flashes, the transient component was abolished and the rod after-effect became larger. The spectral input of the H I cell body was tested with chromatic stimuli matching the spectral sensitivity of primate cone pigments: responses to all wavelengths were hyperpolarizations. Spectral sensitivity values for the different wavelengths and a comparison of the spectral amplitude–intensity curves for 450 nm, 540 nm and 630 nm wavelengths suggested that H I cell bodies had a non-specific, broad-band sensitivity to mid-spectral wavelengths. Thus more than one spectrally sensitive mechanism was involved in the generation of their response and they were not univariant. This was supported by the morphological observation that the clusters of terminal dendritic branchlets of H I cells had the same arrangement and distance from each other as the overlying cone inner segments, suggesting that these cells contact cones in an indiscriminate fashion. The other class of horizontal cell responses were characterized by a lower threshold, a larger amplitude with low intensity stimuli, a steeper increment in amplitude with increasing intensity, a saturation at moderate levels of illumination and a prominent rod after-effect in response to bright-light stimuli. We concluded that these responses were generated by the axon terminal of H I horizontal cells, that receives synaptic input from rods. Thus H I horizontal cells belong to the luminosity type, are not involved in the processing of colour contrast and are homologous to the axon-bearing horizontal cells of other mammalian retinas.