Computer simulation study on local intracellular Ca²⁺ dynamics in the retinal bipolar cell terminal.

Abstract

The bipolar cell of the vertebrate retina gives rise to graded responses to light stimuli and transmits the responses to the inner retinal circuit. The transmitter release from presynaptic terminal is known to be related to the the calcium concentration at the axon terminal ([Ca²⁺]_i). We conducted computer simulations analyses using a biophysical model to elucidate [Ca²⁺]_i change during light-induced responses.
The software NEURON was used for the simulations. L-type and T-type voltage-dependent Ca²⁺ channels (VDCCs), the plasma membrane Ca-ATPase, the intracellular fixed and mobile buffer and diffusion were incorporated in terms of the Ca²⁺-related machineries. Kinetics of these machineries were estimated from previous studies. In the model cell in which L-type and T-type VDCCs were distributed uniformly over the entire cell, [Ca²⁺]_i increased to about 800 nM from about 80 nM in response to a +20 mV and 500 msec depolarization from -40 mV mimicking the voltage range of light-induced responses. This [Ca²⁺]_i level is thought to be inappropriate for the transmitter release. When L-type VDCC was locally distributed at the tip of the terminal region, which may correspond to the Ca²⁺-domain region, and Ca-ATPase was concentrated at the terminal region exclusive of the Ca²⁺-domain, [Ca²⁺]_i at the Ca²⁺-domain increased to more than 80 μM from about 6 μM in response to the same depolarization. This [Ca²⁺]_i level is though to be appropriate for the transmitter release. [Jpn J Physiol 54 Suppl:S160 (2004)]