Abstract
Abstract: :
Purpose:Glycine receptors (GlyR) are pentamers made of α and ß subunits. Retinal neurons express at least three types of α subunits (α 1–3) and one ß subunit. We are attempting to develop a pharmacology of glycine receptor subtypes by comparing responses of retinal neurons with heterologous expression systems containing known glycine subunits. In this study, we examined the responses of glycine subunits to picrotin (a non–toxic component of picrotoxin), and compared this with responses of glycine receptors in retinal neurons. Methods: The amphibian retina was dissociated and acutely isolated third order neurons were identified by morphological and electrophysiological criteria using whole–cell voltage clamp techniques. α1 and α2 subunits, with or without ß subunit, were co–expressed with EGFP in HEK293 cells. Whole–cell recordings were made 1–3 days after transfection. A rapid superfusion system was employed for application of glycine and various antagonists. Results: In third order neurons, picrotin inhibited glycine currents in a dose–dependent manner. 100 µM picrotin blocked 80% of the response to 60 µM glycine (approximate EC50), but did not inhibit GABA currents. When tested in HEK293 cells, picrotin was a much more effective antagonist of α2 GlyR than of α1GlyR. ( IC50 = 7 µM vs 37 µM). The picrotin IC50 was 300 µM in the α1/ß GlyR, but it was 50 µM in the α2/ß GlyR. This indicates that the native GlyR behaves like one made from α2 and ß subunits.We also co–expressed α1 and α2 with ß subunits. The picrotin IC50 of the presumptive α1/α2/ß GlyR was very similar to that of the simpler α2/ß GlyR. This suggests that picrotin can be used to identify native glycine receptors that contain α2 subunits. To test this further, we co–expressed α1 and α2 subunits. The resulting α1/α2 GlyR had a picrotin IC50 of 7 µM, like that of the pure α2 GlyR. This supports the hypothesis that picrotin identifies α2 subunits, even in receptors expressing other α subunits. We also tested several GABA antagonists, including picrotoxin, SR95531 and bicuculline. These antagonists also block glycine receptors in retinal neurons. These inhibitors were very weak glycine antagonists in HEK293 cells expressing α1/ß GlyR (IC50 > 500 µM) but good antagonists at α2/ß GlyR receptors (IC50s ∼ 50 µM), which is similar to their IC50s against glycine in retinal neurons. Conclusions: Picrotin is a particularly useful pharmacological tool because it potently blocks glycine receptors containing α2 subunits but is inactive at the GABA receptor. The studies with picrotin indicate that most GlyR in third order retinal neurons possess α2 subunits, perhaps in combination with other α subunits.
Keywords: inhibitory receptors • retina: proximal (bipolar, amacrine, and ganglion cells) • receptors: pharmacology/physiology