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Defining affinity with the GABAA receptor
M V Jones1, Y Sahara, J A Dzubay
1Vollum Institute, Oregon Health Sciences University, Portland, Oregon 97201, USA.
Summary
GABA receptor selectivity is determined by a ligand-specific energy barrier, not just unbinding rates. This barrier, arising from a flexible binding site, explains differences in agonist binding kinetics.
Area of Science:
- Neuroscience
- Molecular Biology
- Biophysics
Background:
- Postsynaptic response duration is linked to receptor-ligand affinity.
- Receptor-ligand binding is typically assumed to be diffusion-limited, with unbinding rates dictating affinity.
Purpose of the Study:
- To investigate the binding kinetics of GABAA receptors.
- To determine if unbinding rates alone explain agonist affinity.
- To explore the role of binding rates in receptor selectivity.
Main Methods:
- Studied GABAA receptors using outside-out patches from cultured rat hippocampal neurons.
- Applied agonist pulses to measure microscopic binding and unbinding kinetics.
- Used instantaneous competition assays between agonists and antagonists.
Main Results:
- Agonist unbinding rates did not fully account for affinity under diffusion-limited binding assumptions.
- Binding rates were significantly slower than diffusion-limited predictions.
- Binding rates correlated more strongly with affinity than unbinding rates.
Conclusions:
- GABAA receptor selectivity is governed by a ligand-specific energy barrier, not solely diffusion-limited binding.
- This energy barrier arises from a flexible GABA binding site with movable elements.
- Ligand binding kinetics, including rate-limiting steps, are crucial for receptor function.