On the mechanism of partial agonism

Remigijus Lape, Lucia Sivilotti, David Colquhoun
Pharmacology, UCL, London, United Kingdom

A partial agonist is a ligand which, at high enough concentrations to occupy all receptors, can elicit only a relatively small response. In the case of ligand-activated ion channels, it has been supposed ever since 19571 that the basis for the small response lies in the gating reaction, i.e. the change of conformation from an open channel to a shut channel. We investigated partial agonists for two members of the C-loop family, namely, tetramethylammonium (TMA) for the nicotinic acetylcholine receptor and taurine for the glycine receptor.

Single channel currents were recorded from HEK293 cells transfected with wild type acetylcholine receptor α, β, ε and δ or glycine receptor α1 and β subunits in cell-attached configuration. Several mechanisms were fitted by maximising the likelihood of the entire sequence of open and shut time periods, with exact allowance for missed brief events (program hjcfit2). Several records obtained at different agonist concentrations were fitted simultaneously.

We found that the results can be well described by a ‘flip’ mechanism3 in which after binding, the receptor moves through an intermediate shut conformation (‘flip’ state), before the channel opens. For both nicotinic and glycine receptors, full and partial agonists showed very similar gating reactions, so differences in gating were not responsible for partial agonism. Rather, the difference between full and partial agonists originated during the earlier conformation change (flipping) while the channel is still shut. For example the gating constant (the equilibrium constant for the shut-open reaction) is very similar for acetylcholine (34) and TMA (29) but the equilibrium constant for flipping (the change from the resting conformation to the flipped, pre-open, conformation) is much smaller for the partial agonist, TMA (0.12) than for acetylcholine (2.4).

The same idea can be put differently. Rather than agonist efficacy depending on the relative affinity of the agonist for the open conformation and the resting conformation, it seems that what matters is largely the relative affinity of the agonist for the resting receptor and the flipped (still shut) conformation.

This interpretation places the root of partial agonism earlier in the chain of events that follow binding than has been supposed up to now. That is something that might be detectable in structural measurements and could be exploited in rational drug design.

1. Del Castiilo & Katz, 1957 Proc.R.Soc.Lond.B 146, 369-381
2. Colquhoun et al. (2003) J Physiol 547, 699-728.
3. Burzomato et al. (2004) J Neurosci 24, 10924-40.