The glutamate transporter EAAT5 works as a presynaptic receptor in mouse rod bipolar cells

Abstract

Membrane neurotransmitter transporters control the concentration of their substrate in the synaptic clefts, through the thermodynamic coupling of uptake to the movement of Na + and other ions. In addition, excitatory amino acid transporters (EAAT) have a Cl − conductance which is gated by the joint binding of Na + and glutamate, but thermodynamically uncoupled to the flux of glutamate. This conductance is particularly large in the retina-specific EAAT5 isoform. In the mouse retina, we located EAAT5 in both cone and rod photoreceptor terminals and in axon terminals of rod bipolar cells. In these later cells, application of glutamate on the axon terminal evoked a current that reversed at E Cl , was insensitive to bicuculline, TPMPA, strychnine, DL-AP5, CNQX and MCPG, but blocked by the glutamate transporter inhibitor DL-tBOA. Furthermore, short depolarizations of the bipolar cells evoked a DL-tBOA and Cd 2+ -sensitive current whose amplitude was comparable to the glutamate-evoked current. Its kinetics indicated that EAAT5 was located close to the glutamate release site. For 2 ms depolarizations evoking maximal responses, the EAAT5-mediated current carried between 2 and 8 times more charge as an average inhibitory GABA or glycine postsynaptic current received spontaneously from amacrine cells, with 10 mM or 0.5 mM intracellular EGTA, respectively. In conditions for which reciprocal inhibition could be monitored, the charge carried by the EAAT5 current was 1.5 times larger than the one carried by the inhibitory postsynaptic currents received from amacrine cells. These results indicate that EAAT5 acts as a major inhibitory presynaptic receptor at mammalian rod bipolar cell axon terminals. This feedback mechanism could control glutamate release at the ribbon synapses of a non-spiking neuron and increase the temporal contrast in the rod photoreceptor pathway.

Electrical synapses between AII amacrine cells in the retina: Function and modulation

“…This suggests that GluNRs could be involved in modulatory regulation of the electrical synapses and that extrasynaptic receptors could be activated by spillover of glutamate escaping from the synaptic cleft after release from bipolar cells. Veruki et al (2006) found evidence for spillover of glutamate between neighboring rod bipolar cells and that ambient and transiently released glutamate can activate a glutamate transporter on rod bipolar axon terminals (see also Wersinger et al, 2006). This could imply that ambient levels of glutamate might be sufficient to activate GluNRs on AII amacrines and contribute to regulating the junctional conductance of their electrical synapses.…”

Functional Heterogeneity of Retinal Dopaminergic Neurons Underlying Their Multiple Roles in Vision

“…Bistratified bipolar cells make excitatory contacts to DA cells in cat and monkey retinas (Hokoc and Mariani, 1987;Kolb et al, 1990), and in fish retinas, bistratified bipolar cells exhibit L-AP-4-resistant ON light responses mediated by the glutamate-chloride cotransporter EAAT5 (Wong et al, 2005). However, in the mouse retina, EAAT5 localization and function differs from the fish because it is limited to rod, cone, and rod bipolar cell terminals, where it acts primarily to negatively regulate synaptic glutamate release and clear released glutamate (Hasegawa et al, 2006;Wersinger et al, 2006). Application of the transporter inhibitor TBOA causes loss of DA cell spontaneous activity as well as loss of both ON-transient and ON-sustained light responses, effects that are blocked by the glutamate receptor inhibitor CNQX (data not shown).…”

Patch-Clamp Investigations and Compartmental Modeling of Rod Bipolar Axon Terminals in an In Vitro Thin-Slice Preparation of the Mammalian Retina

“…On stepping the cell back to Ϫ60 mV, a relatively long-lasting tail current was observed. This tail-current involves several different response components, including a GABAergic component (Hartveit 1999;Singer and Diamond 2003) and a glutamate transporter-mediated component (Palmer et al 2003;Veruki et al 2006;Wersinger et al 2006). In addition, several discrete IPSCs were evoked (cf.…”