Abstract
Abstract: :
Purpose: The glycine receptor channel (GlyR) belongs to the ligand-gated ion channel superfamily that includes the ionotropic GABA receptor. In the retina, these two receptors are prevalent in the inner retina and are usually differentiated based on the action of antagonists. But the specificity of some antagonists has been questioned. Therefore, studies were performed on retinal neurons and also on glycine receptors expressed in HEK 293 cells to test the specificity of GABA antagonists. Methods: The amphibian retina was dissociated and the acutely isolated third order neurons were studied using whole-cell voltage clamp techniques. Alpha subunits were expressed in HEK 293 cells and were also studied using whole-cell voltage clamp methods. A rapid superfusion system was employed for application of glycine and various antagonists. Results: Picrotoxin, an antagonist of both GABAa and GABAc receptors, inhibited glycine current elicited from retinal third order neurons. The inhibition showed little voltage dependence. SR-95531 and bicuculline, antagonists of the GABAa receptor, inhibited glycine current in third order neurons and the inhibition was voltage dependent. I4AA, an agonist of GABAa receptors but an antagonist of GABAc receptors, had small effects on the glycine current. When alpha subunits were expressed in HEK 293 cells, receptors made of the alpha1 subunit were sensitive to picrotoxin inhibition, but the glycine currents were not blocked by SR-95531 or bicuculline. However, picrotoxin, bicuculline, and SR-95531 inhibited receptors made from alpha2 subunits. Neither expressed subunit was sensitive to I4AA. Conclusions: When GABA antagonists are used at high concentrations sufficient to fully block GABA receptors, then they are also fairly effective antagonists at the glycine receptor. The GABA antagonists can be used to distinguish between different glycine alpha subunits and preliminary evidence suggests that glycine receptor alpha2 subunits are functionally prominent in retinal third order neurons.
Keywords: inhibitory receptors • inhibitory neurotransmitters • electrophysiology: non-clinical