Calcium and synaptic transmission in the retina: the importance of surface charges

The release of synaptic transmitter in chemical synapses is brought about by Ca2+ influx through voltage-dependent Ca2+ channels opened by depolarisation of presynaptic terminals. However, due to persistence or increase of transmitter release in low-Ca2+ media in some preparations, it has been proposed that transmitter release could also occur through a Ca2+-independent, carrier mediated process. From our recent experiments on synaptic transmission in the retina and from a survey of the scientific literature, it appears, however, that lowering extracellular Ca2+ can actually promote calcium influx through voltage-activated Ca2+ channels via a modification of surface potential of plasma membranes. Therefore, the socalled Ca2+-independent release could be reaccommodated within the framework of Ca2+-hypothesis of synaptic transmission by taking into the due account surface charge effects