Morphological and physiological studies of rod-driven horizontal cells with special reference to the question of whether they have axons and axon terminals.
Rod-driven (intermediate) horizontal cells were examined in the carp retina to determine whether they bear axons and axon terminals. These cells were injected with HRP after physiological identification of the response type; which consisted of a higher sensitivity to light and a slower response time course than cone-driven (external) horizontal cells, and spectral sensitivity peaking at 520 nm. The labeled cells were further identified morphologically by tracing their dendrites to rod photoreceptors by light and electron microscopy. About two-thirds of the labeled cells (18/30) had a slender, axonlike process (less than 1 micron in diameter, 70-300 micron in length) running horizontally from the soma. No axonlike process was found in the remaining cells. Unlike the axons of external horizontal cells, this axonlike process was short and did not form a long fusiform expansion. No membrane specialization was found along the axonlike process. Since it has been reported that the syncytium made of axon terminals of external horizontal cells serves as a signal bypass of the syncytium made of the somata, it was asked, in separate experiments, whether the intermediate horizontal cells also had such a double syncytial layer. Response amplitudes to a slit of light were measured by placing the slit at various distances from the recording electrode. The response amplitude decayed with distance with a single exponential function, indicating that the syncytium of intermediate horizontal cells consists of a monolayer. These physiological data are consistent with the morphological observations.
D000818:Animals; D001369:Axons / Q000648:ultrastructure; D002347:Carps / Q000033:anatomy & histology* / Q000502:physiology; D003530:Cyprinidae / Q000033:anatomy & histology*; D008854:Microscopy, Electron; D009474:Neurons / Q000166:cytology; D010786:Photoreceptor Cells / Q000502:physiology; D012160:Retina / Q000166:cytology*