Ionic channels of the inner segment of tiger salamander cone photoreceptors

Cone photoreceptors were isolated enzymatically and their ionic currents studied by the whole-cell, gigaseal voltage-clamp technique. Five nonsynaptic currents were identified. A prominent, poorly selective cation current, Ih, activated after a delay during hyperpolarizations and then deactivated with a delay on return to potentials greater than -50 mV. An empirical model for Ih gating kinetics is developed with three open and two closed states. Depolarization elicits a small, voltage-gated calcium current (ICa). Block by nitrendipine, nickel, cadmium, and cobalt, increase of current with barium, lack of rapid inactivation, and relatively high threshold suggest an L-type Ca channel. No evidence was found for low-threshold Ca channels. An anion current ICl(Ca) was present after pulses that led to a significant inward ICa (but not IBa) and was not elicited when cobalt was present. Tails of ICl(Ca) were short (100 ms) after short depolarizations and were longer after longer depolarizations. Two TEA-sensitive K currents were also elicited by depolarizations. One, IK(Ca), was calcium sensitive. We looked for modulation of Ih, ICa, and ICl(Ca) by a number of neurotransmitters. No changes of Ih were seen, but ICa and ICl(Ca) were depressed in a few cones when GABA or adenosine were applied. We discuss how this modulation might contribute to the feedback effects of horizontal cells on cones when surrounding cones are illuminated.