Pharmacology of Alpha-3-Containing Glycine Receptors

Abstract

Purpose:: Three types of glycine receptor alpha subunit have been detected in retina. They have non-overlapping distributions in the inner plexiform layer, suggesting that each alpha subunit is associated with a particular function. GlyRs containing α1 or α2 subunits have been well characterized. Here we explore the pharmacology of α3 containing GlyRs.

Methods:: Drug effects were tested in retinal and transfected HEK 293 cells using whole cell patch recordings.

Results:: (A) Agonist profile - The α3 GlyR is the least sensitive to glycine, with an EC₅₀ of 166µM and Hill coefficient of 2. Unlike α1 or α2 GlyR, addition of ß subunits made the GlyR even less sensitive to glycine (EC₅₀-396µM). Glycine analogs L-alanine, ß alanine, L-serine, taurine, L-threonine, D-serine, and sarcosine were less effective on α3 than α1 or α2 GlyRs. They were. (B) Antagonist Profile - Picrotoxin (PTX) is a potent blocker of homomeric α3 GlyRs (IC₅₀ 0.86µM). The block was use-dependent and PTX was trapped in the receptor when glycine was withdrawn. PTX blocks α3/ß GlyR with an IC₅₀ of 11.79µM. Thus PTX is a much more potent blocker of α3 GlyRs than either α1 or α2 GlyRs. Picrotin is much more potent at α3/ß GlyR (IC₅₀ 34µM) than at other glycine receptors or retinal GABA receptors (IC₅₀ 90 µM) and may be a prototype for design of specific α3 GlyR antagonists. Picrotoxinin is slightly more effective than picrotin in blocking α3/ß GlyR (IC₅₀ 26µM). 100µM bicuculline elicited a 30% block when tested against EC₅₈ Gly on α3 GlyRs. 20µM SR95531 did not suppress α3 GlyRs, nor did 50 µM TPMPA. (C) Methyl Xanthines - Caffeine blocked both α3 and α3/ß GlyRs, with IC₅₀s of 247µM and 185µM. Theophylline antagonized α3 GlyRs (IC₅₀ of 293µM) as did theobromine (IC₅₀ 450µM). Forskolin and IBMX had no effect.

Conclusions:: GlyRs formed from α3 subunits have distinctive pharmacological profiles. They are less sensitive to glycine than the other subunits yet more sensitive to block by PTX and picrotin. It is fully blocked when PTX is used to inhibit GABA receptors. Like α1 and α2 containing GlyRs, bicuculline partially inhibits α3 GlyR. SR 95531 is the preferred GABA_AR antagonist. Methylxanthines are effective blockers of α3 GlyRs. Unlike GABA antagonists, methylxanthines are more potent in the presence of ß subunits.

Keywords: inhibitory neurotransmitters • ion channels • receptors: pharmacology/physiology