Mechanisms of tonic, graded release: lessons from the vertebrate photoreceptor.

The release of neurotransmitter via exocytosis is a highly conserved, fundamental feature of nervous system function. At conventional synapses, neurotransmitter release occurs as a brief burst of exocytosis triggered by an action potential. By contrast, at the first synapse of the vertebrate visual pathway, not only is the calcium-dependent release of neurotransmitter typically graded with respect to the presynaptic membrane potential, but release can be maintained throughout the duration of a sustained stimulus. The specializations that provide for graded and sustained release are not well-defined. However, recent advances in our understanding of basic synaptic vesicle dynamics and the calcium sensitivity of the release process at these and other central, glutamatergic neurons have shed some light on the photoreceptor's extraordinary abilities.