Electron paramagnetic resonance (EPR) spectroscopy is a powerful technique that permits the study of membrane-embedded
proteins in its
lipid environment by assessing the interaction of
spin labels with the
protein in its natural environment (i.e., native membranes) or in reconstituted systems prepared with exogenous
lipid species.
Nicotinic acetylcholine receptors (AChRs) contain a large surface in intimate contact with the
lipid membrane. AChRs, members of the
Cys-loop receptor superfamily, have essential functional roles in the nervous system and its malfunctioning has been considered as the origin of several neurological diseases including
Alzheimer's disease,
drug addiction, depression, and
schizophrenia. In this regard, these receptors have been extensively studied as therapeutic targets for the action of several drugs. The majority of the marketed medications bind to the
neurotransmitter sites, the so-called agonists. However, several drugs, some of them still in clinical trials, interact with non-competitive antagonist (NCA) binding sites. A potential location for these binding sites is the proper
ion channel, blocking ion flux and thus, inhibiting membrane depolarization. However, several NCAs also bind to the
lipid-
protein interface, modulating the AChR functional properties. The best known examples of these NCAs are local and
general anesthetics. Several endogenous molecules such as
free fatty acids and
neurosteroids also bind to the
lipid-
protein interface, probably mediating important physiological functions.
Phospholipids, natural components of
lipid membranes interacting with the AChR, are also essential to maintain the structural and functional properties of the AChR. EPR studies showed that
local anesthetics bind to the
lipid-
protein interface by essentially the same dynamic mechanisms found in
lipids, and that local and
general anesthetics preferably decrease the
phospholipid but not the
fatty acid interactions with the AChR. This is consistent with the existence of annular and non-annular
lipid domains on the AChR.