The roles of metabotropic GABA receptor in retina circuitry
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GABAergic amacrine cell feedback to bipolar cells has been described, activating both GABA A and GABA C receptors. We explored whether metabotropic GABA B receptors also participate in this feedback pathway. Two approaches were used. One was to identify endogenous, synaptically-activated pathways using GABA B receptor antagonists (CGP55845). The second approach was to investigate the entire array of GABA B receptors using an agonist (baclofen), and compare this to the more delimited synaptically-activated receptors. Ganglion cell EPSCs and IPSCs were monitored to measure the output of bipolar and amacrine cells. Using the tiger salamander slice preparation, we found that synaptically-activated GABA B receptor pathways regulate bipolar cell release directly and indirectly. In the direct pathway, the GABA B receptor antagonist reduces EPSC amplitude, indicating that GABA B receptors cause enhanced glutamate release from bipolar cells to one set of ganglion cells. In the indirect pathway, the GABA B receptor antagonist reduces EPSC amplitude in another set of ganglion cells. The indirect pathway is only evident when GABA A receptors are inhibited, and is blocked by a glycine receptor antagonist. Thus, this second feedback pathway involves direct glycine feedback to the bipolar cell and this glycinergic amacrine cell signal is suppressed by GABAergic amacrine cells, through both GABA A and GABA B , but not GABA C , receptors. Overall, GABA B receptors do contribute to synaptically-activated feedback regulation of bipolar cell transmitter release. However, unlike the ionotropic GABA receptor pathways, the metabotropic GABA receptor pathways act to enhance bipolar cell transmitter release. Furthermore, there are three discrete subsets of bipolar cell output regulated by GABA B receptor feedback (direct, indirect, and null), implying three distinct, non-overlapping bipolar cell to ganglion cell circuits. Interestingly, application of baclofen revealed that activation of the entire set of GABA B receptors produced a novel bimodal control of transmitter release from bipolar cells. Bath application of baclofen activates synaptic and extrasynaptic GABA B Rs. Baclofen inhibited spontaneous and dim light-evoked EPSCs in ganglion cells but concomitantly augmented their responses to bright light stimuli. This contrasts with the synaptic action of GABA B receptors, which was always facilitatory. Curiously, the inhibitory component of GABA B receptor activation was revealed under conditions that favor transmitter spillover. Blocking GABA uptake using NO-711 or drug-induced tonic stimulation of GABAergic amacrine cells, both led to decreases in light-evoked EPSCs that were reversed by the addition of GABA B R antagonists. Furthermore, the effect of blocking of extrasynaptic GIRK channels can also be recovered through application of GABA B R antagonists. Thus, the inhibitory GABA B R appears to be located at extrasynaptic sites that activate potassium channels. Together, these results demonstrate that GABA B Rs are generally facilitatory in the retina, enhancing the glutamate release from bipolar cells. However, GABA B receptors can put a stop to release by activating potassium channels. This will occur if there is excess transmitter release, leading to a "spillover" to extrasynaptic GABA B receptors.