CONE PHOTORECEPTORS RESPOND TO THEIR OWN GLUTAMATE RELEASE IN THE TIGER SALAMANDER

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.