GABAB Receptors Control GABA and Acetylcholine Release in the Direction Selective Pathway

Abstract

Purpose: : The retinal direction selective ganglion cell (DSGC) responds preferentially to motion in one direction. The crucial role in directional selectivity belongs to starburst amacrine cells (SBACs), which release acetylcholine to excite DSGCs, and GABA to inhibit DSGCs and bipolar cells. It has been suggested that the morphological properties of the SBAC generate DS that is amplified by intrinsic or network properties. One possibility for network enhancement of DS is that some bipolar cells are known to express GABA_B receptors, which are negatively coupled to calcium channels, and thus control glutamate release. Other possibilities for enhancement of DS are modulation of GABA/acetylcholine release from SBACs by GABA_B receptors, or glutamate release from bipolar cells by GABA_C receptors. To find out how GABA_B and GABA_C receptors affect direction selectivity, we studied light responses of the DSGC under GABA_B or GABA_C block.

Methods: : We used in vitro whole-mount retinas from Hartley guinea pigs anesthetized with pentobarbital. The retinas (n=5) were maintained in Ames medium at 35ºC. We recorded voltage-clamped responses of On-Off DSGCs to a moving dark bar (40% contrast, 3000µm by 120µm, moving on its longest axis) with velocity 2000 µm/s. We applied conductance analysis to distinguish the excitatory and inhibitory inputs. To block GABA_B receptors, we applied 3 µM CGP52432. To block GABA_C receptors, we applied 50 µM TPMPA. To separate glutamatergic and cholinergic inputs to the DSGC, we blocked nicotinic receptors with 100 µM of hexamethonium and 30 nM methyllycaconitine.

Results: : Block of GABA_B receptors potentiated Off- excitatory and inhibitory inputs to the DSGC by ~2-fold in 3 out of 5 DSGCs, hardly affecting On- excitation and inhibition. To test whether GABA_B receptors directly affected glutamate release from bipolar cells, we blocked GABA_B receptors in the presence of nicotinic receptor antagonists. In this case, GABA_B receptor block moderately decreased (by 20%) both Off- and On- excitatory inputs to the DSGC. In a separate experiment, GABA_C block potentiated excitation to the DSGC by 30% with or without nicotinic block.

Conclusions: : These results imply that GABA_C receptors control glutamatergic excitation to the DSGC, but that GABA_B receptors control its Off- inhibition and cholinergic excitation. This represents an essential asymmetry between On- and Off- direction selective pathways.

Keywords: inhibitory receptors • ganglion cells • amacrine cells