Association of Model Neurotransmitters with Lipid Bilayer Membranes

Abstract

Aimed to reproduce the results of electrophysiological studies of synaptic signal transduction, conventional models of neurotransmission are based on the specific binding of neurotransmitters to ligand-gated receptor ion channels. However, the complex kinetic behavior observed in synaptic transmission cannot be reproduced in a standard kinetic model without the ad hoc postulation of additional conformational channel states. On the other hand, if one invokes unspecific neurotransmitter adsorption to the bilayer-a process not considered in the established models-the electrophysiological data can be rationalized with only the standard set of three conformational receptor states that also depend on this indirect coupling of neurotransmitters via their membrane interaction. Experimental verification has been difficult because binding affinities of neurotransmitters to the lipid bilayer are low. We quantify this interaction with surface plasmon resonance to measure equilibrium dissociation constants in neurotransmitter membrane association. Neutron reflectometry on artificial membranes reveals the structural aspects of neurotransmitters association with zwitterionic and anionic bilayers. We establish that serotonin interacts nonspecifically with the membrane at physiologically relevant concentrations whilst GABA (γaminobutyric acid) does not. Surface plasmon resonance shows that serotonin adsorbs with millimolar affinity and neutron reflectometry shows that it penetrates the membrane deeply whereas GABA is excluded from the bilayer. SignificanceReceptor ion channels in the postsynaptic membrane and their neurotransmitter agonists enable fast communication between neuronal cells. Electrophysiology studies reveal surprisingly complex kinetics that apparently require a variety of protein conformational states for their quantitative interpretation, but an alternate hypothesis invoking neurotransmitter membrane association reduces the complexity of the underlying reaction schemes significantly. While their affinity may be low, and is hard to quantify experimentally, neurotransmitter membrane association can be relevant because of their large temporary concentration in the synaptic cleft. With thermodynamic and structural measurements we quantify membrane-bound states of serotonin, establishing this neurotransmitter as membrane-affine, whereas the affinity of the more hydrophilic GABA is too low to register in our sensitivity-optimized measurement techniques.