CALCIUM-INDEPENDENT RELEASE OF NEUROTRANSMITTER IN THE RETINA - A COPERNICAN VIEWPOINT CHANGE

The release of synaptic transmitter in chemical synapses is brought ab out by Ca2+ influx through voltag-dependent Ca2+ channels opened by de polarisation of presynaptic terminals. However, in some preparations t ransmitter release persists or increases in low-Ca2+ media, and it has therefore been proposed that transmitter release could also occur thr ough a Ca2+-independent, carrier mediated process. In particular it ha s been suggested that this may be the case for synaptic transmission b etween photoreceptors and second order neurones of the vertebrate reti na. From our recent experiments on synaptic transmission from photorec eptors to horizontal cells of turtle and salamander retinas, it appear s that lowering extracellular Ca2+ can actually promote Ca2+ inf lux t hrough voltage-activated Ca2+ channels via a modification of surface p otential of plasma membranes. On the basis of this apparently paradoxi cal effect of low Ca2+ media, Ca2+-independent release within the fram ework of Ca2+-dependent synaptic is possible to reaccommodate the so-c alled Ca2+ transmission without invoking unconventional mechanisms. (C ) 1998 Elsevier Science Ltd. All rights reserved.