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Received December 15, 2002
Accepted after revision March 13, 2003
1 Garvan Institute of Medical Research, 384 Victoria Street, Darlinghurst, Sydney NSW 2010, Australia
2 Garvan Institute of Medical Research, 384 Victoria Street, Darlinghurst, Sydney, NSW 2010 and School of Medicine, University of New South Wales, Sydney NSW 2052, Australia
3 Department of Pharmacology and Toxicology, University of Utah, Salt Lake City, UT 84112, USA
* To whom correspondence should be addressed. E-mail: t.lewis{at}garvan.org.au.
The amino acids glycine, 
-alanine and taurine are all endogenous agonists of the glycine receptor. In this study, a combination of rapid agonist application onto macropatches and steady-state single-channel recordings was used to compare the actions of glycine, -alanine and taurine upon homomeric 
1 human glycine receptors transiently expressed in human embryonic kidney (HEK 293) cells. The 10-90 % rise times determined from rapid application of 100 µM of each agonist were indistinguishable, indicating each agonist has a similar association rate. At saturating concentrations (30 mM) the rise time for glycine (0.26 ms) was 1.8-fold faster than that for -alanine (0.47 ms) and 3.9-fold faster than that for taurine (1.01 ms), indicating clear differences in the maximum opening rate between agonists. The relaxation following rapid removal of agonist was fitted with a single exponential for -alanine (3.0 ms) and taurine (2.2 ms), and two exponential components for glycine with a weighted mean time constant of 27.1 ms. This was consistent with differences in dissociation rates estimated from analysis of bursts, with taurine > -alanine > glycine. Exponential fits to the open period distributions gave time constants that did not differ between agonists and the geometric distribution for the number of openings per burst indicated that all three agonists had a significant component of single-opening bursts. Based upon these data, we propose a kinetic scheme with three independent open states, where the opening rates are dependent upon the activating agonist, while the closing rates are an intrinsic characteristic of the receptor.
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