S. Picaud et al., CONE PHOTORECEPTORS RESPOND TO THEIR OWN GLUTAMATE RELEASE IN THE TIGER SALAMANDER, Proceedings of the National Academy of Sciences of the United Statesof America, 92(20), 1995, pp. 9417-9421
Pulse-like currents resembling miniature postsynaptic currents were re
corded in patch-clamped isolated cones from the tiger salamander retin
a, The events were absent in isolated cones without synaptic terminals
, The frequency of events was increased by either raising the osmotic
pressure or depolarizing the cell. It was decreased by the application
of either glutamate or the glutamate-transport blockers dihydrokainat
e and D,L-threo-3-hydroxyaspartate, The events required external Na+ f
or which Li+ could not substitute. The reversal potential of these cur
rents followed the equilibrium potential for Cl- when internal Cl- con
centration was changed. Thus, these miniature currents appear to repre
sent the presynaptic activation of the glutamate receptor with glutama
te transporter-like pharmacology, caused by the photoreceptor's own ve
sicular glutamate release, Using a noninvasive method to preserve the
intracellular Cl- concentration, we showed that glutamate elicits an o
utward current in isolated cones, Fluorescence of the membrane-permeab
le form of fura-2 was used to monitor Ca2+ entry at the cone terminal
as a measure of membrane depolarization, The increase in intracellular
Ca2+ concentration, elicited by puff application of 30 mM KCI, was co
mpletely suppressed in the presence of 100 mu M glutamate. Puff applic
ation of glutamate alone had no measurable depolarizing effect, These
results suggest that the equilibrium potential for Cl-, E(CI), was mor
e negative than the activation range for Ca2+ channels and that glutam
ate elicited an outward current, hyperpolarizing the cones.