Analysis of Light Responses of the Retinal Bipolar Cells Based on Ionic Current Model
The outer retina which consists of photoreceptor, horizontal cell and bipolar cell is considered as a fundamental neural circuit for spatial and color information processings (Kaneko, 1987). We have developed mathematical models of those cells based on their ionic current mechanisms to analyze the information processings in outer plexiform layer (OPL) of retina (Kamiyama et al., 1996; Usui et al., 1996a, b). In order to further understand information processing in bipolar cells, mathematical models of synaptic connection in OPL have to be constructed. In general, the following sequence of events at the OPL synapse during neurotransmission is involved: 1) At rest in the dark, the synaptic terminal of photoreceptor is depolarized. The depolarization of the terminal activates voltage-gated calcium channels, allowing the entry of calcium ions. 2) The entry of calcium ions into the terminal near the release sites triggers some unknown sequence of events leading to the fusion to the plasma membrane of vesicles containing neurotransmitter glutamates. 3) The glutamates diffuse across the synaptic cleft and make contact with the postsynaptic membrane of bipolar cells. 4) The binding of glutamate to the receptors of the dendrite of bipolar cell causes changes in glutamate induced currents. The release of glutamate is suppressed when photoreceptors are hyperpolarized by light. In the present study, we modeled ionotropic- and metabotropic-type of glutamate induced current of bipolar cell based on available physiological data. We also reconstructed a neural circuit of photoreceptor and bipolar cell by integrating the synaptic models into the model of the bipolar cell body, and analyzed light response properties of a bipolar cell.
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