An intracellular electrophysiological study of the ontogeny of functional synapses in the rabbit retina. I. Receptors, horizontal, and bipolar cells.

An isolated perfused retina-eyecup of the rabbit neonate was used to study the functional maturation of synaptic interactions in the outer plexiform layer. Intracellular recordings from receptors, horizontal cells, hyperpolarizing and depolarizing bipolar cells were obtained in the adult and at different stages of maturation. Three types of horizontal cells could be distinguished based on the relative amount of rod and cone input, response wave form, receptive field diameter, and amplitude-intensity relationships. Two of the horizontal cell types were encountered with sufficient frequency such that maturation of these response characteristics could be followed over the developmental period of this study (8 days-adult). The growth rate of the amplitude-intensity relationships was different for the two commonly encountered types of horizontal cells; the small diameter receptive field type achieved an adultlike amplitude-intensity range at an earlier age than the large field type. Immature bipolar cell responses were initially monophasic potentials with transient-sustained components appearing at a later stage. Center surround antagonism of bipolar cells developed after center-mediated responses were first observed. This suggests some secondary modifications during synaptogenesis are responsible for the late maturation of center surround antagonism. The morphological appearance of synaptic contacts during different stages of synaptogenesis is discussed in reference to the different phases of functional, synaptic interactions of this study. It appears that some synapses in the vertebrate retina are functional at a time when synaptic structure is incomplete, based on ultrastructural observation. Maturation of receptive field diameter and amplitude-intensity function is discussed in relationship to possible presynaptic and postsynaptic mechanisms which may influence this growth.