We examined the functional properties of a low-voltage-activated (LVA)
calcium current in ganglion cells of the neotenous tiger salamander
(Ambystoma tigrinum) retina. Our analysis was based on whole-cell
recordings from acutely dissociated ganglion cell bodies identified by
retrograde dye injections. Using a continuously perfused cell preparation,
the LVA current was isolated with the use of potassium channel blocking
agents added to the bathing medium and the pipette solution, while
tetrodotoxin was added to the bathing medium to block Na+
channels. Approximately 70% of ganglion cells had an easily identified LVA
current. The LVA current activated at membrane potentials more positive
than −90 mV, and inactivated rapidly. It was relatively insensitive
to nickel (IC50 > 500 μM) and amiloride (IC50 > 750 μM).
Voltage- and current-clamp studies allowed us to generate a model of this
current using the NEURON simulation program. Studies were also carried out
to measure the LVA Ca2+ current in ganglion cells with
dendrites to confirm that it had a significant dendritic representation.
Physiological mechanisms that may depend on LVA Ca2+ currents
are discussed with an emphasis on the role that dendrites play in ganglion
cell function.