Low-voltage activated calcium currents in ganglion cells of the tiger salamander retina: experiment and simulation.

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 microM) and amiloride (IC50 > 750 microM). 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.