The glutamate-activated current in photoreceptors has been attributed
both to a sodium/glutamate transporter and to a glutamate-activated ch
loride channel. We have further studied the glutamate-activated curren
t in single, isolated photoreceptors from the tiger salamander using n
oise analysis on whole-cell patch-clamp recordings. In cones, the curr
ent is generated by chloride channels with a single-channel conductanc
e of 0.7 pS and an open lifetime of 2.4 ms. The number of channels per
cell is in the range of 10,000-20,000. Activation of the channels req
uires the presence of both glutamate and sodium. The single-channel co
nductance and the open lifetime of the channel are independent of the
external concentration of glutamate and sodium. External glutamate and
sodium affect only the opening rate of the channels. D,L-Threo-3-hydr
oxyaspartate (THA), a glutamate-transport blocker, is shown to be a pa
rtial agonist for the channel. The single-channel conductance is the s
ame regardless of whether glutamate or THA is the ligand, but the open
lifetime of the channel is only 0.8 ms with THA as ligand. The glutam
ate-activated current in rods has a similar single-channel conductance
(0.74 pS) and open lifetime (3 ms). We propose a kinetic model, consi
stent with these results, to explain how a transporter can simultaneou
sly act both as a sodium/glutamate-gated chloride channel and a glutam
ate/sodium cotransporter.