Incorporation of the nicotinic acetylcholine receptor into planar multilamellar films: characterization by fluorescence and Fourier transform infrared difference spectroscopy.

A method for preparing thin, planar films of nicotinic acetylcholine receptor (nAChR) membranes that retain the ability to undergo the resting to desensitized state transition and that are suitable for spectroscopic studies has been developed. Native, alkaline-extracted nAChR membranes from Torpedo are dried under nitrogen on either a plastic microscope coverslip or a germanium internal reflection element (IRE) and then equilibrated with buffer. The drying procedure has no effect on the functional state of the nAChR as judged by a fluorescence assay using the probe ethidium bromide. The times required for an acetylcholine analogue (carbamylcholine), a local anesthetic (dibucaine), and a fluorescent probe (ethidium bromide) to penetrate films of varying degrees of thickness, interact with the receptor, and then to be washed from the films have been established. Under these conditions, the nAChR films can be repetitively cycled between the resting and desensitized states. Both fluorescence and infrared spectroscopy show that the films adhere strongly to either support even with buffer flowing continuously past the film surface. Fourier transform infrared difference spectra calculated from spectra recorded in the presence and absence of carbamylcholine show small, reproducible bands which reflect changes in nAChR structure upon desensitization.