Ratfish retina-intracellular recordings and HRP injections in an isolated, superfused all-rod retina

Hyperpolarizing responses to light were studied by intracellular recording in the isolated, superfused retina of the ratfish (Hydrolagus colliei). Two of these hyperpolarizing units were identified as horizontal cells by iontophoretic injection of horseradish peroxidase (HRP). These cells had rather large cell bodies (15 × 30 μm), elliptical dendritic arborizations measuring 150 × 300 μm and no axons. Since their physiological receptive fields were found to be at least 2.15 mm in diameter, it appears likely that either the photoreceptors or the horizontal cells are electrically coupled. Electron microscopy of HRP-injected horizontal cells showed their dendrites to end laterally in ribbon synapses of rods only and revealed dendro-dendritic contacts resembling gap junctions between injected and uninjected horizontal cells. The spectral sensitivity function of a dark-adapted horizontal cell can be described by a Dartnall nomogram based on a retinene₁ pigment with λ_max = 473 nm. These findings are consistent with the histological observation that the ratfish retina appears to contain only rod photoreceptors.