Dynamics of intracellular free calcium concentration in the presynaptic arbors of individual barnacle photoreceptors.

At photoreceptor synapses, transmitter release is continuous and graded. At this type of synapse, the control of presynaptic [Ca2+]i and calcium's role in releasing transmitter might be different than at terminals invaded by all-or-none action potentials. To examine this possibility, we measured the spatial and temporal changes of [Ca2+]i in response to depolarization of individual photoreceptor terminals of the barnacle Balanus nubilus, which had been injected with the Ca2+ indicator Fura-2. Depolarizing pulses produced voltage-dependent Ca2+ entry that was confined to the tips of the arbor where the release sites are located. At increasing distances from the tips, the rate of [Ca2+]i increase was slower and the peak [Ca2+]i occurred later, suggesting that Ca2+ entered the tips and diffused back into the larger processes of the arbor. Consistent with this result, a stable gradient of [Ca2+]i was observed at maintained depolarizations, with the highest values at the tips of the arbor. Removal of external Na+ did not affect the time course of Ca2+ decline in the terminal, indicating that Na+/Ca2+ exchange was not the primary mechanism for restoring [Ca2+]i to basal levels. Computer simulations, assuming only Ca2+ entry at the arbor's tips and diffusion of Ca2+ away from the entry site, qualitatively reproduced these observations. The threshold for Ca2+ entry was near -60 mV, and entry was maintained during prolonged depolarizations, in agreement with previous experiments showing that Ca2+ channels in the terminal region do not inactivate. The time course of the measured [Ca2+]i change in the terminal paralleled voltage changes due to a Ca(2+)-activated K+ conductance, which senses [Ca2+]i just under the membrane. This parallelism is expected since the release sites are located on processes of small-enough diameter to permit radial equilibration of [Ca2+]i within the time course of physiological voltage changes. Therefore, the optical measurements reflect the mean level of [Ca2+]i under the membrane. Whether this mean concentration is also the value at the sites that trigger exocytosis will depend on how close the Ca2+ channels are to these sites.