Physical structure of the excitable membrane of unmyelinated axons: X-ray scattering study and electrophysiological properties of pike olfactory nerve

The aim of this work was to elicit correlations between physical structure and physiological functions in excitable membranes. Freshly dissected pike olfactory nerves were studied by synchrotron radiation X-ray scattering exp eriments and their physiological properties were tested by electrophysiolog ical techniques. The scattering spectra contained a sharply oriented equato rial component (i.e. normal to the nerve axis), and an isotropic background . After background subtraction, the equatorial component displayed a weak a nd fairly sharp spectrum of oriented microtubules, and a strong and diffuse band of almost the same shape and position as the band computed for an iso lated myelin membrane. We ascribed this spectrum to the axonal membranes. U nder the action of temperature and of two local anesthetics, the spectrum u nderwent a contraction (or expansion) in the s-direction, equivalent to the structure undergoing an expansion (or contraction) in the direction perpen dicular to the plane of the membrane. The main observations were: (i) with increasing temperature, membrane thickness decreased with a thermal expansi on coefficient equal to -0.97(+/-0.19) 10(-3) degreesC(-1). The polarity an d amplitude of this coefficient are typical of lipid-containing systems wit h the hydrocarbon chains in a disordered conformation. The amplitude and pr opagation velocity of the compound action potentials were drastically and r eversibly reduced by lowering the temperature from 20 degreesC to 5 degrees C. (ii) Exposing the nerve to two local anesthetics (tetracaine and dibucai ne) had the effect of decreasing membrane thickness. Action potentials were fully inhibited by these anesthetics. (iii) Upon depolarization, induced b y replacing NaCl with KCl in the outer medium, approximately 25% of the mem branes were found to associate by apposing their outer faces. Electrophysio logical activity was reversibly impaired by the KCl treatment. (iv) No dete ctable structural effect was observed upon exposing the nerves to tetrodoto xin or veratridine. Electrophysiological activity was fully impaired by tet rodotoxin and partially impaired by veratridine. The main conclusions of th is work are that axonal membranes yield highly informative X-ray scattering spectra, and that these spectra are sensitive to the functional state of t he nerve. These results pave the way to further studies of more direct phys iological significance. (C) 2000 Academic Press.