Modulation of transmitter release via presynaptic ligand-gated ion channels

Neurons communicate through the exocytotic release of transmitters from presynaptic axon terminals and the ensuing activation of postsynaptic receptors. Instantaneous responses of postsynaptic cells to released neurotransmitters are mediated by ligand-gated ion channels, whereas G protein-coupled receptors mediate rather delayed effects. Moreover, the actions of ionotropic receptors are transient (milliseconds to seconds) and those of G protein-coupled receptors are more long lasting (seconds to minutes). Accordingly, neuronal signalling via ligand-gated ion channels is termed neurotransmission, whereas signalling via G protein-coupled receptors is termed neuromodulation. Exocytotic transmitter release is modulated by a variety of mechanisms such as previous activity at the synapse and the presence of extracellular neurotransmitters. Like the postsynaptic responses, presynaptic modulation is not only mediated by slowly acting G protein-coupled receptors, but also by fast acting ligand-gated ion channels. Accordingly, members of all known families of ligand-gated ion channels (cys-loop receptors, such as GABA(A), glycine, nicotinic acetylcholine, and 5-HT(3) receptors, ionotropic glutamate receptors, P2X receptors, and vanilloid receptors) are known to control transmitter release. All these ligand-gated ion channels display heterogeneous structures and functions. Therefore, activation of such presynaptic receptors can control transmitter release in different ways and through a multitude of mechanisms. This review provides a summary of the functions of the different presynaptic ligand-gated ion channels and presents prototypic examples for the physiological and pharmacological relevance of these presynaptic receptors.