Influence of Ca2+ on synaptic morphology of fish cone photoreceptors

Isolated fish retinas were incubated with the Ca(2+)-ionophore A23187 at various Ca2+ concentrations. Computer reconstructions of the ultrastructure of cone synapses revealed pronounced reduction of the presynaptic surface (complete loss of the synaptic invaginations) in response to low intracellular Ca2+ (less than 10(-8) M). No obvious effect on synaptic morphology was noticed at Ca2+ greater than 10(-8) M. Ca(2+)-dependent exocytotic activity appears to be a major factor controlling the invaginated morphology of cone synapses.