Prolonged Reciprocal Signaling via NMDA and GABA Receptors at a Retinal Ribbon Synapse

Abstract

AMPA and GABA A receptors mediate most of the fast signaling in the CNS. However, the retina must, in addition, also convey slow and sustained signals. Given that AMPA and GABA A receptors desensitize quickly in the continuous presence of agonist, how are sustained excitatory and inhibitory signals transmitted reliably across retinal synapses? Reciprocal synapses between bipolar and amacrine cells in the retina are thought to play a fundamental role in tuning the bipolar cell output to the dynamic range of ganglion cells. Here, we report that glutamate release from goldfish bipolar cell terminals activates first AMPA receptors, followed by fast and transient GABA Amediated feedback. Subsequently, prolonged NMDA receptor activation triggers GABA A and a slow, sustained GABA C -mediated reciprocal inhibition. The synaptic delay of the NMDA/GABA C -mediated feedback showed stronger dependence on the depolarization of the bipolar cell terminal than the fast AMPA/GABA A -mediated response. Although the initial depolarization mediated by AMPA receptors was important to prime the NMDA action, NMDA receptors could trigger feedback by themselves in most of the bipolar terminals tested. This AMPA-independent feedback (delay Ϸ 10 ms) was eliminated in 2 mM external Mg 2ϩ and reduced in some terminals, but not eliminated, by TTX. NMDA receptors on amacrine cells with depolarized resting membrane potentials therefore can mediate the late reciprocal feedback triggered by continuous glutamate release. Our findings suggest that the characteristics of NMDA receptors (high agonist affinity, slow desensitization, and activation/deactivation kinetics) are well suited to match the properties of GABA C receptors, which thus provide part of the prolonged inhibition to bipolar cell terminals.

Calcium spike-mediated digital signaling increases glutamate output at the visual threshold of retinal bipolar cells

“…Supporting this notion are the findings that GABA puff eliminated spontaneous Ca 2ϩ spikes in solitary, cultured Mbs (Zenisek and Matthews 1998) and that the magnitude of lateral and reciprocal GABAergic feedback to Mb1 terminals in retina slice preparation showed a great variability (Vigh et al 2011;Vigh and von Gersdorff 2005). However, blocking inhibitory GABAergic signaling by blockade of GABA A/C receptors with 100 M PTX (Vigh et al 2011;Vigh and von Gersdorff 2005) could not switch spiking light responses to graded ones and did not turn graded responses into spikes (Fig. 4, Bi and Bii, respectively).…”

“…1). In these experiments and every experiment presented from this point on, the GABA-mediated chloride current associated with reciprocal feedback from amacrine cells to depolarized Mb terminals was blocked with PTX (100 M) (Vigh et al 2011;Vigh and von Gersdorff 2005).…”

“…In both internal solutions, the concentration of EGTA was 2 mM, approximating the buffering capacity of soluble endogenous Ca 2ϩ buffers (Burrone et al 2002). In voltageclamp recordings, Ames medium was supplemented with picrotoxin (PTX, 100 M) to block GABA-gated chloride conductances in Mb terminals (Vigh et al 2011;Vigh and von Gersdorff 2005).…”

Active Roles of Electrically Coupled Bipolar Cell Network in the Adult Retina

“…However, asynchronous release from the Mb1-BC was evident only after the termination of a long (ϳ200 ms) depolarization (von Gersdorff et al, 1998), and thus, it seems unlikely that the brief depolarization (20 ms) could exhibit asynchronous release strong enough to evoke the long-lasting EPSC. Delayed activation of NMDA receptors in GCs (Matsui et al, 1998;Vigh and von Gersdorff, 2005) would also contribute to the longlasting EPSC. However, it seems unlikely because NMDA receptors could be mostly blocked by extracellular Mg 2ϩ at the holding potential of Ϫ60 mV (Nowak et al, 1984), and that application of D-AP5 did not change the duration of the longlasting EPSC (data not shown).…”

“…It was likely that the evoked EPSC shown in Figure 6A was modulated by inhibitory amacrine cells, i.e., inhibitory feedback to the Mb1-BC and/or inhibitory feedforward to the postsynaptic GC (Lukasiewicz and Werblin, 1990;Han et al, 1997;Matsui et al, 2001;Vigh and von Gersdorff, 2005). Blockade of inhibitory synaptic transmission by GABA-and glycine-receptor blockers (picrotoxin and strychnine, respectively) reduced the outward current in the Mb1-BC (Fig.…”