Ligand-gated ion channels

So far in this volume, the various aspects of voltage-gated ion channels have been discussed. Opening and closing (gating) of voltage-gated ion channels is strictly dependent on changes in membrane polarisation although many additional factors may modify this dependency. The ligand-gated ion channels constitute another main class of ion channel. In this type of transporter, gating occurs only after binding of specific compounds to the channel protein. These compounds are called ‘ligands’, i.e. they bind to another compound to form a complex ( Fig. 7.1 ; Berg et al ., 2001 ). The term ‘agonist’ (literally meaning a ‘competitor’) is also frequently used, since often different ligands can have similar actions and compete for the same binding site. In contrast, ‘antagonists’ also bind at the same site but have an opposite action to ligands. Ligand binding to the channel protein causes a conformational change that switches the channel from the closed to the open state or vice versa. Binding of ligand is a minimum requirement for channel gating but, as for voltage-gated channels, overall gating properties of ligand-gated channels are often modulated by additional parameters, which can include membrane polarisation ( Aidley & Stanfield, 1996 ; Hille, 2001 ).