Association of Model Neurotransmitters with Lipid Bilayer Membranes

Abstract

The conventional model of synaptic transmission between neurons is based on the specific binding of neurotransmitters to ligand-gated ion channels. Fast perfusion electrophysiological studies of receptor responses to neurotransmitters have revealed complex kinetic behavior that cannot be reproduced unless the standard kinetic model is expanded to include additional conformational states. However, if one invokes neurotransmitter adsorption to the lipid membrane, the electrophysiological data can be reproduced with a simpler kinetic model that includes only the standard set of three conformational states [1]. This indirect mechanism of influence of neurotransmitters on receptor conformational transitions is assumed to be nonspecific. Unlike anesthetics, experimental verification has been difficult because of the low binding affinities of neurotransmitters to lipid bilayers [2]. We quantify this interaction by measuring the equilibrium dissociation constant of neurotransmitters on membranes with surface plasmon resonance (SPR) spectroscopy and characterize neurotransmitter association with bilayers through neutron reflectometry (NR) on artificial membranes. Sparsely-tethered bilayer lipid membranes (stBLMs) composed of zwitterionic (PC) and anionic (PS and PG) lipids were assembled and their interactions with serotonin and γ-aminobutyric acid (GABA) were studied as model systems. SPR shows a range of binding affinities for different neurotransmitters. Consistent with these results, NR shows that the ligand with the largest affinity (serotonin) penetrates the membrane deeply whereas GABA, for which the affinity is a tenth of serotonin, associates with the bilayer peripherally. Overall, we establish that some neurotransmitters interact non-specifically with the lipidic membrane matrix at physiologically relevant concentrations and that this interaction differs vastly for different neurotransmitters. These results could have a significant impact on our understanding of the molecular mechanism of synaptic transmission. 1. Sonner and Cantor, Annu. Rev. Biophys. 42, 143 (2013). 2. Wang, et al., J. Phys. Chem. B. 155, 196 (2011).