LOW-CALCIUM-INDUCED ENHANCEMENT OF CHEMICAL SYNAPTIC TRANSMISSION FROM PHOTORECEPTORS TO HORIZONTAL CELLS IN THE VERTEBRATE RETINA

According to the classical calcium hypothesis of synaptic transmission , the release of neurotransmitter from presynaptic terminals occurs th rough an exocytotic process triggered by depolarization-induced presyn aptic calcium influx. However, evidence has been accumulating in the l ast two decades indicating that, in many preparations, synaptic transm itter release can persist or even increase when calcium is omitted fro m the perfusing saline, leading to the notion of a ''calcium-independe nt release'' mechanism. Our study shows that the enhancement of synapt ic transmission between photoreceptors and horizontal cells of the ver tebrate retina induced by low-calcium media is caused by an increase o f calcium influx into presynaptic terminals. This paradoxical effect i s accounted for by modifications of surface potential on the photorece ptor membrane. Since lowering extracellular calcium concentration may likewise enhance calcium influx into other nerve cells, other experime ntal observations of ''calcium-independent'' release may be reaccommod ated within the framework of the classical calcium hypothesis without invoking unconventional processes.