Aimed at reproducing 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 reflection measurements on artificial membranes, so-called sparsely tethered bilayer lipid membranes, reveal the structural aspects of neurotransmitters' association with zwitterionic and anionic bilayers. We thus establish that serotonin interacts nonspecifically with the membrane at physiologically relevant concentrations, whereas γ-aminobutyric acid does not. Surface plasmon resonance shows that serotonin adsorbs with millimolar affinity, and neutron reflectometry shows that it penetrates the membrane deeply, whereas γ-aminobutyric is excluded from the bilayer.
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