A glutamate transporter in photoreceptor terminals plays a crucial role in controlling the kinetics and magnitude of light responses in postsynaptic cells; as light hyperpolarizes photoreceptors, the transporter removes glutamate from the synaptic cleft. We have studied the properties of the transporter in the rod photoreceptor and its role in regulating synaptic transmission by measuring transporter activity in rods, and its effects on postsynaptic cells.; Under voltage clamp, application of L-glutamate generated an inwardly rectifying current in rods. The current could be elicited by bath application of L-aspartate, D-aspartate, L-cysteic acid and D-glutamate, and could be reduced by the glutamate transporter antagonists dihydrokainate (DHKA) and {dollar}beta{dollar}-hydroxy aspartate ({dollar}beta{dollar}HA). The glutamate receptor agonists kainate, quisqualate, NMDA and APB generated no current. The current could not be blocked by the glutamate receptor antagonists CNQX and APV. This pharmacology is consistent with a transporter-mediated current.; The current requires extracellular but not intracellular sodium, nor extracellular or intracellular potassium or calcium. Changing the intracellular chloride concentration shifted the reversal potential of the current. Removal of extracellular chloride abolished the outward component of the current. Reducing the extracellular chloride concentration did not significantly shift the current's reversal potential. The current could not be blocked by the chloride-channel blockers DIDS, SITS, Picrotoxin and 9-AC.; The role of the transporter in synaptic transmission was evaluated by recording from horizontal cells under conditions that altered the effectiveness of the transporter. {dollar}beta{dollar}HA was used to block glutamate transport in both rods and cones. When glutamate transport was completely blocked, horizontal cells depolarized strongly and their light responses were lost, suggesting that without the transporter the concentration of glutamate rises to saturating levels. When the transporter was only partially blocked the onset of the horizontal cell light response was slowed, demonstrating that the rapid onset of the light response is mediated by a normally-robust glutamate transporter system. The concentration of glutamate at the dendrites of postsynaptic cells thus appears to be regulated by a dynamic balance between vesicular release and transporter uptake at photoreceptor terminals.
展开▼
