Calcium pathways and synaptic transmission: An investigation of the rod synapse
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The thesis discusses the role of calcium-induced-calcium-release (CICR) from RYR-gated ER calcium stores during synaptic transmission at retinal synapses, focusing on the photoreceptor synapses. In the first section, inhibitor of the ryanodine receptor (RYR), ryanodine (100 μM) was used to demonstrate that store-based calcium amplification plays an important role during synaptic transmission at the rod photoreceptor synapse. Inhibiting RYR-gated calcium amplification in rods suppresses light-evoked synaptic transmission, frequency of spontaneous mEPSCs as well as depolarization-evoked release from rods. RYR-based amplification combines with calcium influx to initiate and extend glutamate release at the dark resting potential of the rod where calcium influx alone might be relatively small. RYR-gated calcium release might work by producing a spread of calcium to gate release of a large cytoplasmic pool. During desynchronized release long after stimulation, RYR-gated calcium amplification promotes larger postsynaptic events at higher frequency, suggesting a role in coordinating vesicle release at the synapse. Stimulators of RYR-gated calcium release such as the methylxanthines, caffeine (10 mM) and IBMX (100 μM), were used in the second section to initiate transmission in the dark in a RYR-dependent fashion. In dark-adapted conditions, the rod is capable of further release of transmitter on application of caffeine or IBMX. They initiate fast glutamate release by depolarizing the rod membrane potential eventually producing a presynaptic calcium-dependent spike. Transmission at the rod synapse occurs via interaction between glutamate release, re-uptake via glutamate transporters and fast postsynaptic AMPA receptor kinetics. Desensitization of AMPA receptors is not prominent during synaptically evoked release of glutamate in rod-dominated OFF bipolar cells perhaps due to low cleft glutamate concentrations maintained by a transporter mechanism and due to fast postsynaptic AMPA receptor kinetics. The kinetics of the light response is different in cone-dominated OFF bipolar cells. In effect caffeine and IBMX were used as effective stimulators of rod/cone synapse to characterize properties of synaptic transmission at retinal synapse.