A computational model of the ribbon synapse in bipolar retinal cells

Abstract

The bipolar cells, horizontal and amacrine cells communicate with the ganglion cells via synaptic contacts. Bipolar cells, along with amacrine and horizontal cells, have the role to control the detection of light by graded impulse responses of glutamate which are released by the photoreceptors. The ganglion cells are depolarized by synaptic glutamatergic release from bipolar cells. The synaptic transmission depends on the level of intracellular calcium concentration in the bipolar cells. In this thesis, the influence of the membrane potential over the calcium channel dynamics and calcium current was simulated and analyzed. Calcium ion flux changes its direction from inside to outside of the cell in regions where the voltage membrane exceeds the equilibrium potential of calcium. A computational study was performed in order to get a better insight of the roll of the bipolar cell in the transduced signal. Furthermore, it was aimed to investigate the intracellular calcium concentration in the bipolar cell terminals using voltage clamp or space clamp technique. The intracellular calcium concentration has the main role in triggering neurotransmitter release and in this purpose it is analyzed how it changes during a stimulus of different wave-forms, with different amplitudes and time duration. In the last section, the impact of different intracellular calcium concentrations on the vesicle release was investigated. The discrete synaptic vesicle release is simulated with a four pool model and it doesn't include any endocytosis mechanism because the simulations were performed on short terms. In this study, L-type voltage-gated calcium channels was used to explain calcium current in type 9 ON bipolar cell in the rat retina. The results of this thesis may give an insight on how various computer simulations could resemble physiologic process in retinal bipolar cells. In this scope, the influence of sinusoidal stimuli of different frequencies (from 20 Hz to 5 kHz) and amplitudes (30 μA to 50 μA) or by using repetitive rectangular pulses was investigated.